# Using the pyparsing module

author:

  Paul McGuire

address:

  ptmcg.pm+pyparsing@gmail.com

revision:

  3.2.0

date:

  October, 2024

copyright:

  Copyright © 2003-2024 Paul McGuire.

abstract:

  This document provides how-to instructions for the pyparsing library, an easy-to-use Python module for constructing and executing basic text parsers. The pyparsing module is useful for evaluating user-definable expressions, processing custom application language commands, or extracting data from formatted reports.

Note: While this content is still valid, there are more detailed descriptions and extensive examples at the online doc server, and in the online help for the various pyparsing classes and methods (viewable using the Python interpreter’s built-in `help()` function). You will also find many example scripts in the examples directory of the pyparsing GitHub repo.

Note: In pyparsing 3.0, many method and function names which were originally written using camelCase have been converted to PEP8-compatible snake_case. So ``parseString()`` is being renamed to ``parse_string()``, ``delimitedList`` to DelimitedList_, and so on. You may see the old names in legacy parsers, and they will be supported for a time with synonyms, but the synonyms will be removed in a future release.

If you are using this documentation, but working with a 2.4.x version of pyparsing, you’ll need to convert methods and arguments from the documented snake_case names to the legacy camelCase names. In pyparsing 3.x, both forms are supported, but the legacy forms are deprecated; they will be dropped in a future 4.0 release.

## Steps to follow

To parse an incoming data string, the client code must follow these steps:

1. First define the tokens and patterns to be matched, and assign this to a program variable. Optional results names or parse actions can also be defined at this time.

2. Call `parse_string()`, `scan_string()`, or `search_string()` on this variable, passing in the string to be parsed. During the matching process, whitespace between tokens is skipped by default (although this can be changed). When token matches occur, any defined parse action methods are called.

3. Process the parsed results, returned as a ParseResults object. The ParseResults object can be accessed as if it were a list of strings. Matching results may also be accessed as named attributes of the returned results, if names are defined in the definition of the token pattern, using `set_results_name()`.

### Hello, World!

The following complete Python program will parse the greeting `"Hello, World!"`, or any other greeting of the form “<salutation>, <addressee>!”:

    import pyparsing as pp

    greet = pp.Word(pp.alphas) + "," + pp.Word(pp.alphas) + "!"
    for greeting_str in [
                "Hello, World!",
                "Bonjour, Monde!",
                "Hola, Mundo!",
                "Hallo, Welt!",
            ]:
        greeting = greet.parse_string(greeting_str)
        print(greeting)

The parsed tokens are returned in the following form:

    ['Hello', ',', 'World', '!']
    ['Bonjour', ',', 'Monde', '!']
    ['Hola', ',', 'Mundo', '!']
    ['Hallo', ',', 'Welt', '!']

### Usage notes

- The pyparsing module can be used to interpret simple command strings or algebraic expressions, or can be used to extract data from text reports with complicated format and structure (“screen or report scraping”). However, it is possible that your defined matching patterns may accept invalid inputs. Use pyparsing to extract data from strings assumed to be well-formatted.

- To keep up the readability of your code, use operators such as `+`, `|`, `^`, and `~` to combine expressions. You can also combine string literals with `ParseExpressions` - they will be automatically converted to Literal objects. For example:

      integer  = Word(nums)            # simple unsigned integer
      variable = Char(alphas)          # single letter variable, such as x, z, m, etc.
      arith_op = one_of("+ - * /")      # arithmetic operators
      equation = variable + "=" + integer + arith_op + integer    # will match "x=2+2", etc.

  In the definition of `equation`, the string `"="` will get added as a `Literal("=")`, but in a more readable way.

- The pyparsing module’s default behavior is to ignore whitespace. This is the case for 99% of all parsers ever written. This allows you to write simple, clean, grammars, such as the above `equation`, without having to clutter it up with extraneous `ws` markers. The `equation` grammar will successfully parse all of the following statements:

      x=2+2
      x = 2+2
      a = 10   *   4
      r= 1234/ 100000

  Of course, it is quite simple to extend this example to support more elaborate expressions, with nesting with parentheses, floating point numbers, scientific notation, and named constants (such as `e` or `pi`). See fourFn.py, and simpleArith.py included in the examples directory.

- To modify pyparsing’s default whitespace skipping, you can use one or more of the following methods:

  - use the static method `ParserElement.set_default_whitespace_chars` to override the normal set of whitespace chars (`' \t\n'`). For instance when defining a grammar in which newlines are significant, you should call `ParserElement.set_default_whitespace_chars(' \t')` to remove newline from the set of skippable whitespace characters. Calling this method will affect all pyparsing expressions defined afterward.

  - call `leave_whitespace()` on individual expressions, to suppress the skipping of whitespace before trying to match the expression

  - use `Combine` to require that successive expressions must be adjacent in the input string. For instance, this expression:

        real = Word(nums) + '.' + Word(nums)

    will match “3.14159”, but will also match “3 . 12”. It will also return the matched results as [‘3’, ‘.’, ‘14159’]. By changing this expression to:

        real = Combine(Word(nums) + '.' + Word(nums))

    it will not match numbers with embedded spaces, and it will return a single concatenated string ‘3.14159’ as the parsed token.

- Repetition of expressions can be indicated using `*` or `[]` notation. An expression may be multiplied by an integer value (to indicate an exact repetition count), or indexed with a tuple, representing min and max repetitions (with `...` representing no min or no max, depending whether it is the first or second tuple element). See the following examples, where n is used to indicate an integer value:

  - `expr*3` is equivalent to `expr + expr + expr`

  - `expr[2, 3]` is equivalent to `expr + expr + Opt(expr)`

  - `expr[n, ...]` or `expr[n,]` is equivalent to `expr*n + ZeroOrMore(expr)` (read as “at least n instances of expr”)

  - `expr[... ,n]` is equivalent to `expr*(0, n)` (read as “0 to n instances of expr”)

  - `expr[...]`, `expr[0, ...]` and `expr * ...` are equivalent to `ZeroOrMore(expr)`

  - `expr[1, ...]` is equivalent to `OneOrMore(expr)`

  Note that `expr[..., n]` does not raise an exception if more than n exprs exist in the input stream; that is, `expr[..., n]` does not enforce a maximum number of expr occurrences. If this behavior is desired, then write `expr[..., n] + ~expr`.

- `[]` notation will also accept a stop expression using ‘:’ slice notation:

  - `expr[...:end_expr]` is equivalent to `ZeroOrMore(expr, stop_on=end_expr)`

- MatchFirst expressions are matched left-to-right, and the first match found will skip all later expressions within, so be sure to define less-specific patterns after more-specific patterns. If you are not sure which expressions are most specific, use Or expressions (defined using the `^` operator) - they will always match the longest expression, although they are more compute-intensive.

- Or expressions will evaluate all of the specified subexpressions to determine which is the “best” match, that is, which matches the longest string in the input data. In case of a tie, the left-most expression in the Or list will win.

- If parsing the contents of an entire file, pass it to the `parse_file` method using:

      expr.parse_file(source_file)

- `ParseExceptions` will report the location where an expected token or expression failed to match. For example, if we tried to use our “Hello, World!” parser to parse “Hello World!” (leaving out the separating comma), we would get an exception, with the message:

      pyparsing.ParseException: Expected "," (6), (1,7)

  In the case of complex expressions, the reported location may not be exactly where you would expect. See more information under ParseException .

- Use the `Group` class to enclose logical groups of tokens within a sublist. This will help organize your results into more hierarchical form (the default behavior is to return matching tokens as a flat list of matching input strings).

- Punctuation may be significant for matching, but is rarely of much interest in the parsed results. Use the `suppress()` method to keep these tokens from cluttering up your returned lists of tokens. For example, DelimitedList matches a succession of one or more expressions, separated by delimiters (commas by default), but only returns a list of the actual expressions - the delimiters are used for parsing, but are suppressed from the returned output.

- Parse actions can be used to convert values from strings to other data types (ints, floats, booleans, etc.).

- Results names are recommended for retrieving tokens from complex expressions. It is much easier to access a token using its field name than using a positional index, especially if the expression contains optional elements. You can also shortcut the `set_results_name` call:

      stats = ("AVE:" + real_num.set_results_name("average")
               + "MIN:" + real_num.set_results_name("min")
               + "MAX:" + real_num.set_results_name("max"))

  can more simply and cleanly be written as this:

      stats = ("AVE:" + real_num("average")
               + "MIN:" + real_num("min")
               + "MAX:" + real_num("max"))

- Be careful when defining parse actions that modify global variables or data structures (as in fourFn.py), especially for low level tokens or expressions that may occur within an And expression; an early element of an And may match, but the overall expression may fail.

## Classes

All the pyparsing classes can be found in this UML class diagram.

### Classes in the pyparsing module

`ParserElement` - abstract base class for all pyparsing classes; methods for code to use are:

- `parse_string(source_string, parse_all=False)` - only called once, on the overall matching pattern; returns a ParseResults object that makes the matched tokens available as a list, and optionally as a dictionary, or as an object with named attributes; if `parse_all` is set to True, then `parse_string` will raise a ParseException if the grammar does not process the complete input string.

- `parse_file(source_file)` - a convenience function, that accepts an input file object or filename. The file contents are passed as a string to `parse_string()`. `parse_file` also supports the `parse_all` argument.

- `scan_string(source_string)` - generator function, used to find and extract matching text in the given source string; for each matched text, returns a tuple of:

  - matched tokens (packaged as a ParseResults object)

  - start location of the matched text in the given source string

  - end location in the given source string

  `scan_string` allows you to scan through the input source string for random matches, instead of exhaustively defining the grammar for the entire source text (as would be required with `parse_string`).

- `transform_string(source_string)` - convenience wrapper function for `scan_string`, to process the input source string, and replace matching text with the tokens returned from parse actions defined in the grammar (see set_parse_action).

- `search_string(source_string)` - another convenience wrapper function for `scan_string`, returns a list of the matching tokens returned from each call to `scan_string`.

- `set_name(name)` - associate a short descriptive name for this element, useful in displaying exceptions and trace information

- `run_tests(tests_string)` - useful development and testing method on expressions, to pass a multiline string of sample strings to test against the expression. Comment lines (beginning with `#`) can be inserted and they will be included in the test output:

      digits = Word(nums).set_name("numeric digits")
      real_num = Combine(digits + '.' + digits)
      real_num.run_tests("""\
          # valid number
          3.14159

          # no integer part
          .00001

          # no decimal
          101

          # no decimal value
          101.
          """)

  will print:

      # valid number
      3.14159
      ['3.14159']

      # no integer part
      .00001
      ^
      FAIL: Expected numeric digits, found '.'  (at char 0), (line:1, col:1)

      # no decimal
      101
         ^
      FAIL: Expected ".", found end of text  (at char 3), (line:1, col:4)

      # no decimal value
      101.
          ^
      FAIL: Expected numeric digits, found end of text  (at char 4), (line:1, col:5)

- `set_results_name(string, list_all_matches=False)` - name to be given to tokens matching the element; if multiple tokens within a repetition group (such as ZeroOrMore or DelimitedList) the default is to return only the last matching token - if `list_all_matches` is set to True, then a list of all the matching tokens is returned.

  `expr.set_results_name("key")` can also be written `expr("key")` (a results name with a trailing ‘*’ character will be interpreted as setting `list_all_matches` to `True`).

  Note: `set_results_name` returns a copy of the element so that a single basic element can be referenced multiple times and given different names within a complex grammar.

- `using_each(list_of_symbols)` a short-cut for defining a number of symbols of a particular `ParserElement` subclass:

      LBRACK, RBRACK, LBRACE, RBRACE, LPAR, RPAR = Suppress.using_each("[]{}()")
      AND, OR, NOT = Keyword.using_each("and or not".split())

- `set_parse_action(*fn)` - specify one or more functions to call after successful matching of the element; each function is defined as `fn(s, loc, toks)`, where:

  - `s` is the original parse string

  - `loc` is the location in the string where matching started

  - `toks` is the list of the matched tokens, packaged as a ParseResults object

  Parse actions can have any of the following signatures:

      fn(s: str, loc: int, tokens: ParseResults)
      fn(loc: int, tokens: ParseResults)
      fn(tokens: ParseResults)
      fn()

  Multiple functions can be attached to a `ParserElement` by specifying multiple arguments to `set_parse_action`, or by calling `add_parse_action`. Calls to `set_parse_action` will replace any previously defined parse actions. `set_parse_action(None)` will clear all previously defined parse actions.

  Each parse action function can return a modified `toks` list, to perform conversion, or string modifications. For brevity, `fn` may also be a lambda - here is an example of using a parse action to convert matched integer tokens from strings to integers:

      int_number = Word(nums).set_parse_action(lambda s, l, t: [int(t[0])])

  If `fn` modifies the `toks` list in-place, it does not need to return and pyparsing will use the modified `toks` list.

  If `set_parse_action` is called with an argument of `None`, then this clears all parse actions attached to that expression.

  A nice short-cut for calling `set_parse_action` is to use it as a decorator:

      identifier = Word(alphas, alphanums + "_")

      @identifier.set_parse_action
      def resolve_identifier(results: ParseResults):
          return variable_values.get(results[0])

  (Posted by @MisterMiyagi in this SO answer: https://stackoverflow.com/a/63031959/165216)

- `add_parse_action` - similar to `set_parse_action`, but instead of replacing any previously defined parse actions, will append the given action or actions to the existing defined parse actions.

- `add_condition` - a simplified form of `add_parse_action` if the purpose of the parse action is to simply do some validation, and raise an exception if the validation fails. Takes a method that takes the same arguments, but simply returns `True` or `False`. If `False` is returned, an exception will be raised.

- `set_break(break_flag=True)` - if `break_flag` is `True`, calls `pdb.set_break()` as this expression is about to be parsed

- `copy()` - returns a copy of a `ParserElement`; can be used to use the same parse expression in different places in a grammar, with different parse actions attached to each; a short-form `expr()` is equivalent to `expr.copy()`

- `leave_whitespace()` - change default behavior of skipping whitespace before starting matching (mostly used internally to the pyparsing module, rarely used by client code)

- `set_whitespace_chars(chars)` - define the set of chars to be ignored as whitespace before trying to match a specific `ParserElement`, in place of the default set of whitespace (space, tab, newline, and return)

- `set_default_whitespace_chars(chars)` - class-level method to override the default set of whitespace chars for all subsequently created ParserElements (including copies); useful when defining grammars that treat one or more of the default whitespace characters as significant (such as a line-sensitive grammar, to omit newline from the list of ignorable whitespace)

- `suppress()` - convenience function to suppress the output of the given element, instead of wrapping it with a `Suppress` object.

- `ignore(expr)` - function to specify parse expression to be ignored while matching defined patterns; can be called repeatedly to specify multiple expressions; useful to specify patterns of comment syntax, for example

- `set_debug(flag=True)` - function to enable/disable tracing output when trying to match this element

- `validate()` - function to verify that the defined grammar does not contain infinitely recursive constructs.

  (``validate()`` is deprecated, and will be removed in a future pyparsing release. Pyparsing now supports left-recursive parsers, which this function attempted to catch.)

- `parse_with_tabs()` - function to override default behavior of converting tabs to spaces before parsing the input string; rarely used, except when specifying whitespace-significant grammars using the White class.

- `enable_packrat()` - a class-level static method to enable a memoizing performance enhancement, known as “packrat parsing”. packrat parsing is disabled by default, since it may conflict with some user programs that use parse actions. To activate the packrat feature, your program must call the class method `ParserElement.enable_packrat()`. For best results, call `enable_packrat()` immediately after importing pyparsing.

- `enable_left_recursion()` - a class-level static method to enable pyparsing with left-recursive (LR) parsers. Similar to `ParserElement.enable_packrat()`, your program must call the class method `ParserElement.enable_left_recursion()` to enable this feature. `enable_left_recursion()` uses a separate packrat cache, and so is incompatible with `enable_packrat()`.

### Basic ParserElement subclasses

- `Literal` - construct with a string to be matched exactly

- `CaselessLiteral` - construct with a string to be matched, but without case checking; results are always returned as the defining literal, NOT as they are found in the input string

- `Keyword` - similar to Literal, but must be immediately followed by whitespace, punctuation, or other non-keyword characters; prevents accidental matching of a non-keyword that happens to begin with a defined keyword

- `CaselessKeyword` - similar to Keyword, but with caseless matching behavior as described in CaselessLiteral.

- `Word` - one or more contiguous characters; construct with a string containing the set of allowed initial characters, and an optional second string of allowed body characters; for instance, a common `Word` construct is to match a code identifier - in C, a valid identifier must start with an alphabetic character or an underscore (‘_’), followed by a body that can also include numeric digits. That is, `a`, `i`, `MAX_LENGTH`, `_a1`, `b_109_`, and `plan9FromOuterSpace` are all valid identifiers; `9b7z`, `$a`, `.section`, and `0debug` are not. To define an identifier using a `Word`, use either of the following:

      Word(alphas+"_", alphanums+"_")
      Word(srange("[a-zA-Z_]"), srange("[a-zA-Z0-9_]"))

  Pyparsing also provides pre-defined strings `identchars` and `identbodychars` so that you can also write:

      Word(identchars, identbodychars)

  If only one string given, it specifies that the same character set defined for the initial character is used for the word body; for instance, to define an identifier that can only be composed of capital letters and underscores, use one of:

      ``Word("ABCDEFGHIJKLMNOPQRSTUVWXYZ_")``
      ``Word(srange("[A-Z_]"))``

  A `Word` may also be constructed with any of the following optional parameters:

  - `min` - indicating a minimum length of matching characters

  - `max` - indicating a maximum length of matching characters

  - `exact` - indicating an exact length of matching characters; if `exact` is specified, it will override any values for `min` or `max`

  - `as_keyword` - indicating that preceding and following characters must be whitespace or non-keyword characters

  - `exclude_chars` - a string of characters that should be excluded from init_chars and body_chars

    Sometimes you want to define a word using all characters in a range except for one or two of them; you can do this with the `exclude_chars` argument. This is helpful if you want to define a word with all `printables` except for a single delimiter character, such as ‘.’. Previously, you would have to create a custom string to pass to Word. With this change, you can just create `Word(printables, exclude_chars='.')`.

- `Char` - a convenience form of `Word` that will match just a single character from a string of matching characters:

      single_digit = Char(nums)

- `CharsNotIn` - similar to Word, but matches characters not in the given constructor string (accepts only one string for both initial and body characters); also supports `min`, `max`, and `exact` optional parameters.

- `Regex` - a powerful construct, that accepts a regular expression to be matched at the current parse position; accepts an optional `flags` parameter, corresponding to the flags parameter in the `re.compile` method; if the expression includes named sub-fields, they will be represented in the returned ParseResults.

- `QuotedString` - supports the definition of custom quoted string formats, in addition to pyparsing’s built-in `dbl_quoted_string` and `sgl_quoted_string`. `QuotedString` allows you to specify the following parameters:

  - `quote_char` - string of one or more characters defining the quote delimiting string

  - `esc_char` - character to escape quotes, typically backslash (default=None)

  - `esc_quote` - special quote sequence to escape an embedded quote string (such as SQL’s “” to escape an embedded “) (default=None)

  - `multiline` - boolean indicating whether quotes can span multiple lines (default=False)

  - `unquote_results` - boolean indicating whether the matched text should be unquoted (default=True)

  - `end_quote_char` - string of one or more characters defining the end of the quote delimited string (default=None => same as `quote_char`)

- `SkipTo` - skips ahead in the input string, accepting any characters up to the specified pattern; may be constructed with the following optional parameters:

  - `include` - if set to true, also consumes the match expression (default is false)

  - `ignore` - allows the user to specify patterns to not be matched, to prevent false matches

  - `fail_on` - if a literal string or expression is given for this argument, it defines an expression that should cause the SkipTo expression to fail, and not skip over that expression

  `SkipTo` can also be written using `...`:

      LBRACE, RBRACE = Literal.using_each("{}")

      brace_expr = LBRACE + SkipTo(RBRACE) + RBRACE
      # can also be written as
      brace_expr = LBRACE + ... + RBRACE

- `White` - also similar to Word, but matches whitespace characters. Not usually needed, as whitespace is implicitly ignored by pyparsing. However, some grammars are whitespace-sensitive, such as those that use leading tabs or spaces to indicating grouping or hierarchy. (If matching on tab characters, be sure to call parse_with_tabs on the top-level parse element.)

- `Empty` - a null expression, requiring no characters - will always match; useful for debugging and for specialized grammars

- `NoMatch` - opposite of `Empty`, will never match; useful for debugging and for specialized grammars

### Expression subclasses

- `And` - construct with a list of `ParserElements`, all of which must match for `And` to match; can also be created using the ‘+’ operator; multiple expressions can be `Anded` together using the ‘*’ operator as in:

      ip_address = Word(nums) + ('.' + Word(nums)) * 3

  A tuple can be used as the multiplier, indicating a min/max:

      us_phone_number = Word(nums) + ('-' + Word(nums)) * (1,2)

  A special form of `And` is created if the ‘-’ operator is used instead of the ‘+’ operator. In the `ip_address` example above, if no trailing ‘.’ and `Word(nums)` are found after matching the initial `Word(nums)`, then pyparsing will back up in the grammar and try other alternatives to `ip_address`. However, if `ip_address` is defined as:

      strict_ip_address = Word(nums) - ('.'+Word(nums))*3

  then no backing up is done. If the first `Word(nums)` of `strict_ip_address` is matched, then any mismatch after that will raise a `ParseSyntaxException`, which will halt the parsing process immediately. By careful use of the ‘-’ operator, grammars can provide meaningful error messages close to the location where the incoming text does not match the specified grammar.

- `Or` - construct with a list of `ParserElements`, any of which must match for `Or` to match; if more than one expression matches, the expression that makes the longest match will be used; can also be created using the ‘^’ operator

- `MatchFirst` - construct with a list of `ParserElements`, any of which must match for `MatchFirst` to match; matching is done left-to-right, taking the first expression that matches; can also be created using the ‘|’ operator

- `Each` - similar to And, in that all of the provided expressions must match; however, `Each` permits matching to be done in any order; can also be created using the ‘&’ operator

- `Opt` - construct with a `ParserElement`, but this element is not required to match; can be constructed with an optional `default` argument, containing a default string or object to be supplied if the given optional parse element is not found in the input string; parse action will only be called if a match is found, or if a default is specified.

  An optional element `expr` can also be expressed using `expr | ""`.

  (`Opt` was formerly named `Optional`, but since the standard Python library module `typing` now defines `Optional`, the pyparsing class has been renamed to `Opt`. A compatibility synonym `Optional` is defined, but will be removed in a future release.)

- `ZeroOrMore` - similar to `Opt`, but can be repeated; `ZeroOrMore(expr)` can also be written as `expr[...]`.

- `OneOrMore` - similar to ZeroOrMore, but at least one match must be present; `OneOrMore(expr)` can also be written as `expr[1, ...]`.

- `DelimitedList` - used for matching one or more occurrences of `expr`, separated by `delim`. By default, the delimiters are suppressed, so the returned results contain only the separate list elements. Can optionally specify `combine=True`, indicating that the expressions and delimiters should be returned as one combined value (useful for scoped variables, such as `"a.b.c"`, or `"a::b::c"`, or paths such as `"a/b/c"`). Can also optionally specify `min` and ``max` restrictions on the length of the list, and `allow_trailing_delim` to accept a trailing delimiter at the end of the list.

- `FollowedBy` - a lookahead expression, requires matching of the given expressions, but does not advance the parsing position within the input string

- `NotAny` - a negative lookahead expression, prevents matching of named expressions, does not advance the parsing position within the input string; can also be created using the unary ‘~’ operator

### Expression operators

- `+` - creates And using the expressions before and after the operator

- `|` - creates MatchFirst (first left-to-right match) using the expressions before and after the operator

- `^` - creates Or (longest match) using the expressions before and after the operator

- `&` - creates Each using the expressions before and after the operator

- `*` - creates And by multiplying the expression by the integer operand; if expression is multiplied by a 2-tuple, creates an And of `(min,max)` expressions (similar to `{min,max}` form in regular expressions); if `min` is `None`, interpret as `(0,max)`; if `max` is `None`, interpret as `expr*min + ZeroOrMore(expr)`

- `-` - like `+` but with no backup and retry of alternatives

- `~` - creates NotAny using the expression after the operator

- `==` - matching expression to string; returns `True` if the string matches the given expression

- `<<=` - inserts the expression following the operator as the body of the `Forward` expression before the operator (`<<` can also be used, but `<<=` is preferred to avoid operator precedence misinterpretation of the pyparsing expression)

- `...` - inserts a SkipTo expression leading to the next expression, as in `Keyword("start") + ... + Keyword("end")`.

- `[min, max]` - specifies repetition similar to `*` with `min` and `max` specified as the minimum and maximum number of repetitions. `...` can be used in place of `None`. For example `expr[...]` is equivalent to `ZeroOrMore(expr)`, `expr[1, ...]` is equivalent to `OneOrMore(expr)`, and `expr[..., 3]` is equivalent to “up to 3 instances of `expr`”.

- `[:stop_on]` - specifies a stopping expression for the current repetition (may be combined with `...` or `min, max`), as in `Keyword("start") + Word(alphas)[...:Keyword("end")] + Keyword("end")`

### Positional subclasses

- `StringStart` - matches beginning of the text

- `StringEnd` - matches the end of the text

- `LineStart` - matches beginning of a line (lines delimited by `\n` characters)

- `LineEnd` - matches the end of a line

- `WordStart` - matches a leading word boundary

- `WordEnd` - matches a trailing word boundary

### Converter subclasses

- `Combine` - joins all matched tokens into a single string, using specified `join_string` (default `join_string=""`); expects all matching tokens to be adjacent, with no intervening whitespace (can be overridden by specifying `adjacent=False` in constructor)

- `Suppress` - clears matched tokens; useful to keep returned results from being cluttered with required but uninteresting tokens (such as list delimiters)

### Special subclasses

- `Group` - causes the matched tokens to be enclosed in a list; useful in repeated elements like ZeroOrMore and OneOrMore to break up matched tokens into groups for each repeated pattern

- `Dict` - like `Group`, but also constructs a dictionary, using the `[0]`’th elements of all enclosed token lists as the keys, and each token list as the value

- `Forward` - placeholder token used to define recursive token patterns; when defining the actual expression later in the program, insert it into the `Forward` object using the `<<=` operator (see fourFn.py for an example).

- `Tag` - a non-parsing token that always matches, and inserts a tag and value into the current parsed tokens; useful for adding metadata or annotations to parsed results (see examples/tag_example.py).

### Other classes

- `ParseResults` - class used to contain and manage the lists of tokens created from parsing the input using the user-defined parse expression. `ParseResults` can be accessed in a number of ways:

  - as a list

    - total list of elements can be found using `len()`

    - individual elements can be found using `[0], [1], [-1],` etc., or retrieved using slices

    - elements can be deleted using `del`

    - the last element can be extracted and removed in a single operation using `pop()`, or any element can be extracted and removed using `pop(n)`

    - a nested ParseResults can be created by using the pyparsing `Group` class around elements in an expression:

          Word(alphas) + Group(Word(nums)[...]) + Word(alphas)

      will parse the string “abc 100 200 300 end” as:

          ['abc', ['100', '200', '300'], 'end']

      If the `Group` is constructed using `aslist=True`, the resulting tokens will be a Python list instead of a ParseResults. In this case, the returned value will no longer support the extended features or methods of a ParseResults.

  - as a dictionary

    - if `set_results_name()` is used to name elements within the overall parse expression, then these fields can be referenced as dictionary elements or as attributes

    - the `Dict` class generates dictionary entries using the data of the input text - in addition to ParseResults listed as `[ [ a1, b1, c1, ...], [ a2, b2, c2, ...] ]` it also acts as a dictionary with entries defined as `{ a1 : [ b1, c1, ... ] }, { a2 : [ b2, c2, ... ] }`; this is especially useful when processing tabular data where the first column contains a key value for that line of data; when constructed with `asdict=True`, will return an actual Python `dict` instead of a ParseResults. In this case, the returned value will no longer support the extended features or methods of a ParseResults.

    - list elements that are deleted using `del` will still be accessible by their dictionary keys

    - supports `get()`, `items()` and `keys()` methods, similar to a dictionary

    - a keyed item can be extracted and removed using `pop(key)`. Here `key` must be non-numeric (such as a string), in order to use dict extraction instead of list extraction.

    - new named elements can be added (in a parse action, for instance), using the same syntax as adding an item to a dict (`parse_results["X"] = "new item"`); named elements can be removed using `del parse_results["X"]`

  - as a nested list

    - results returned from the Group class are encapsulated within their own list structure, so that the tokens can be handled as a hierarchical tree

  - as an object

    - named elements can be accessed as if they were attributes of an object: if an element is referenced that does not exist, it will return `""`.

  ParseResults can also be converted to an ordinary list of strings by calling `as_list()`. Note that this will strip the results of any field names that have been defined for any embedded parse elements. (The `pprint` module is especially good at printing out the nested contents given by `as_list()`.)

  If a ParseResults is built with expressions that use results names (see _set_results_name) or using the `Dict` class, then those names and values can be extracted as a Python dict using `as_dict()`.

  Finally, ParseResults can be viewed by calling `dump()`. `dump()` will first show the `as_list()` output, followed by an indented structure listing parsed tokens that have been assigned results names.

  Here is sample code illustrating some of these methods:

      >>> number = Word(nums)
      >>> name = Combine(Word(alphas)[...], adjacent=False, join_string=" ")
      >>> parser = number("house_number") + name("street_name")
      >>> result = parser.parse_string("123 Main St")
      >>> print(result)
      ['123', 'Main St']
      >>> print(type(result))
      <class 'pyparsing.ParseResults'>
      >>> print(repr(result))
      (['123', 'Main St'], {'house_number': ['123'], 'street_name': ['Main St']})
      >>> result.house_number
      '123'
      >>> result["street_name"]
      'Main St'
      >>> result.as_list()
      ['123', 'Main St']
      >>> result.as_dict()
      {'house_number': '123', 'street_name': 'Main St'}
      >>> print(result.dump())
      ['123', 'Main St']
      - house_number: '123'
      - street_name: 'Main St'

### Exception classes and Troubleshooting

- `ParseException` - exception returned when a grammar parse fails; `ParseExceptions` have attributes `loc`, `msg`, `line`, `lineno`, and `column`; to view the text line and location where the reported ParseException occurs, use:

      except ParseException as err:
          print(err.line)
          print(" " * (err.column - 1) + "^")
          print(err)

  `ParseExceptions` also have an `explain()` method that gives this same information:

      except ParseException as err:
          print(err.explain())

- `RecursiveGrammarException` - exception returned by `validate()` if the grammar contains a recursive infinite loop, such as:

      bad_grammar = Forward()
      good_token = Literal("A")
      bad_grammar <<= Opt(good_token) + bad_grammar

- `ParseFatalException` - exception that parse actions can raise to stop parsing immediately. Should be used when a semantic error is found in the input text, such as a mismatched XML tag.

- `ParseSyntaxException` - subclass of `ParseFatalException` raised when a syntax error is found, based on the use of the ‘-’ operator when defining a sequence of expressions in an And expression.

- You can also get some insights into the parsing logic using diagnostic parse actions, and `set_debug()`, or test the matching of expression fragments by testing them using `search_string()` or `scan_string()`.

- Use `with_line_numbers` from `pyparsing_testing` to display the input string being parsed, with line and column numbers that correspond to the values reported in set_debug() output:

      import pyparsing as pp
      ppt = pp.testing

      data = """\
         A
            100"""

      expr = pp.Word(pp.alphanums).set_name("word").set_debug()
      print(ppt.with_line_numbers(data))
      expr[...].parse_string(data)

  prints:

      .          1
        1234567890
      1:   A|
      2:      100|

      Match word at loc 3(1,4)
          A
          ^
      Matched word -> ['A']
      Match word at loc 11(2,7)
             100
             ^
      Matched word -> ['100']

  with_line_numbers has several options for displaying control characters, end-of-line and space markers, Unicode symbols for control characters - these are documented in the function’s docstring.

- Diagnostics can be enabled using `pyparsing.enable_diag` and passing one of the following enum values defined in `pyparsing.Diagnostics`

  - `warn_multiple_tokens_in_named_alternation` - flag to enable warnings when a results name is defined on a MatchFirst or Or expression with one or more And subexpressions

  - `warn_ungrouped_named_tokens_in_collection` - flag to enable warnings when a results name is defined on a containing expression with ungrouped subexpressions that also have results names

  - `warn_name_set_on_empty_Forward` - flag to enable warnings when a `Forward` is defined with a results name, but has no contents defined

  - `warn_on_parse_using_empty_Forward` - flag to enable warnings when a `Forward` is defined in a grammar but has never had an expression attached to it

  - `warn_on_assignment_to_Forward` - flag to enable warnings when a `Forward` is defined but is overwritten by assigning using `'='` instead of `'<<='` or `'<<'`

  - `warn_on_multiple_string_args_to_oneof` - flag to enable warnings when `one_of` is incorrectly called with multiple str arguments

  - `enable_debug_on_named_expressions` - flag to auto-enable debug on all subsequent calls to `ParserElement.set_name`

  All warnings can be enabled by calling `pyparsing.enable_all_warnings()`. Sample:

      import pyparsing as pp
      pp.enable_all_warnings()

      fwd = pp.Forward().set_results_name("recursive_expr")

      >>> UserWarning: warn_name_set_on_empty_Forward: setting results name 'recursive_expr'
                       on Forward expression that has no contained expression

  Warnings can also be enabled using the Python `-W` switch (using `-Wd` or `-Wd:::pyparsing`) or setting a non-empty value to the environment variable `PYPARSINGENABLEALLWARNINGS`. (If using `-Wd` for testing, but wishing to disable pyparsing warnings, add `-Wi:::pyparsing`.)

## Miscellaneous attributes and methods

### Helper methods

- `counted_array(expr)` - convenience function for a pattern where an list of instances of the given expression are preceded by an integer giving the count of elements in the list. Returns an expression that parses the leading integer, reads exactly that many expressions, and returns the array of expressions in the parse results - the leading integer is suppressed from the results (although it is easily reconstructed by using len on the returned array).

- `one_of(choices, caseless=False, as_keyword=False)` - convenience function for quickly declaring an alternative set of Literal expressions. `choices` can be passed as a list of strings or as a single string of values separated by spaces. The values are sorted so that longer matches are attempted first; this ensures that a short value does not mask a longer one that starts with the same characters. If `caseless=True`, will create an alternative set of CaselessLiteral tokens. If `as_keyword=True`, `one_of` will declare Keyword expressions instead of Literal expressions.

- `dict_of(key, value)` - convenience function for quickly declaring a dictionary pattern of `Dict(ZeroOrMore(Group(key + value)))`.

- `make_html_tags(tag_str)` and `make_xml_tags(tag_str)` - convenience functions to create definitions of opening and closing tag expressions. Returns a pair of expressions, for the corresponding `<tag>` and `</tag>` strings. Includes support for attributes in the opening tag, such as `<tag attr1="abc">` - attributes are returned as named results in the returned ParseResults. `make_html_tags` is less restrictive than `make_xml_tags`, especially with respect to case sensitivity.

- `infix_notation(base_operand, operator_list)` - convenience function to define a grammar for parsing infix notation expressions with a hierarchical precedence of operators. To use the `infix_notation` helper:

  1. Define the base “atom” operand term of the grammar. For this simple grammar, the smallest operand is either an integer or a variable. This will be the first argument to the `infix_notation` method.

  2. Define a list of tuples for each level of operator precedence. Each tuple is of the form `(operand_expr, num_operands, right_left_assoc, parse_action)`, where:

     - `operand_expr` - the pyparsing expression for the operator; may also be a string, which will be converted to a Literal; if `None`, indicates an empty operator, such as the implied multiplication operation between ‘m’ and ‘x’ in “y = mx + b”.

     - `num_operands` - the number of terms for this operator (must be 1, 2, or 3)

     - `right_left_assoc` is the indicator whether the operator is right or left associative, using the pyparsing-defined constants `OpAssoc.RIGHT` and `OpAssoc.LEFT`.

     - `parse_action` is the parse action to be associated with expressions matching this operator expression (the `parse_action` tuple member may be omitted)

  3. Call `infix_notation` passing the operand expression and the operator precedence list, and save the returned value as the generated pyparsing expression. You can then use this expression to parse input strings, or incorporate it into a larger, more complex grammar.

  Here is an `infix_notation` definition for 4-function arithmetic, taking numbers or variables as operands. The order of definition of the operators follows the standard precedence of operations for arithmetic:

      number = pp.common.number()
      variable = pp.common.identifier()
      arithmetic_expression = pp.infix_notation(
          integer | variable,
          [
              ("-", 1, pp.OpAssoc.RIGHT),
              (pp.one_of("* /"), 2, pp.OpAssoc.LEFT),
              (pp.one_of("+ -"), 2, pp.OpAssoc.LEFT),
          ]
      )

  `infix_notation` also supports optional arguments `lpar` and `rpar`, to parse groups with symbols other than “(” and “)”. They may be passed as strings (in which case they will be converted to `Suppress` objects, and suppressed from the parsed results), or passed as pyparsing expressions, in which case they will be kept as-is, and grouped with their contents.

  For instance, to use “<” and “>” for grouping symbols, you could write:

      expr = infix_notation(int_expr,
          [
              (one_of("+ -"), 2, OpAssoc.LEFT),
          ],
          lpar="<",
          rpar=">"
          )
      expr.parse_string("3 - <2 + 11>")

  returning:

      [3, '-', [2, '+', 11]]

  If the grouping symbols are to be retained, then pass them as pyparsing `Literals`:

      expr = infix_notation(int_expr,
          [
              (one_of("+ -"), 2, OpAssoc.LEFT),
          ],
          lpar=Literal("<"),
          rpar=Literal(">")
          )
      expr.parse_string("3 - <2 + 11>")

  returning:

      [3, '-', ['<', [2, '+', 11], '>']]

- `match_previous_literal` and `match_previous_expr` - function to define an expression that matches the same content as was parsed in a previous parse expression. For instance:

      first = Word(nums)
      match_expr = first + ":" + match_previous_literal(first)

  will match “1:1”, but not “1:2”. Since this matches at the literal level, this will also match the leading “1:1” in “1:10”.

  In contrast:

      first = Word(nums)
      match_expr = first + ":" + match_previous_expr(first)

  will not match the leading “1:1” in “1:10”; the expressions are evaluated first, and then compared, so “1” is compared with “10”.

- `nested_expr(opener, closer, content=None, ignore_expr=quoted_string)` - method for defining nested lists enclosed in opening and closing delimiters.

  - `opener` - opening character for a nested list (default=”(“); can also be a pyparsing expression

  - `closer` - closing character for a nested list (default=”)”); can also be a pyparsing expression

  - `content` - expression for items within the nested lists (default=None)

  - `ignore_expr` - expression for ignoring opening and closing delimiters (default=``quoted_string``)

  If an expression is not provided for the content argument, the nested expression will capture all whitespace-delimited content between delimiters as a list of separate values.

  Use the `ignore_expr` argument to define expressions that may contain opening or closing characters that should not be treated as opening or closing characters for nesting, such as `quoted_string` or a comment expression. Specify multiple expressions using an Or or MatchFirst. The default is `quoted_string`, but if no expressions are to be ignored, then pass `None` for this argument.

- `IndentedBlock(statement_expr, recursive=False, grouped=True)` - function to define an indented block of statements, similar to indentation-based blocking in Python source code:

  - `statement_expr` - the expression defining a statement that will be found in the indented block; a valid `IndentedBlock` must contain at least 1 matching `statement_expr`

  - `recursive` - flag indicating whether the IndentedBlock can itself contain nested sub-blocks of the same type of expression (default=False)

  - `grouped` - flag indicating whether the tokens returned from parsing the IndentedBlock should be grouped (default=True)

- `original_text_for(expr)` - helper function to preserve the originally parsed text, regardless of any token processing or conversion done by the contained expression. For instance, the following expression:

      full_name = Word(alphas) + Word(alphas)

  will return the parse of “John Smith” as [‘John’, ‘Smith’]. In some applications, the actual name as it was given in the input string is what is desired. To do this, use `original_text_for`:

      full_name = original_text_for(Word(alphas) + Word(alphas))

- `ungroup(expr)` - function to “ungroup” returned tokens; useful to undo the default behavior of And to always group the returned tokens, even if there is only one in the list.

- `lineno(loc, string)` - function to give the line number of the location within the string; the first line is line 1, newlines start new rows

- `col(loc, string)` - function to give the column number of the location within the string; the first column is column 1, newlines reset the column number to 1

- `line(loc, string)` - function to retrieve the line of text representing `lineno(loc, string)`; useful when printing out diagnostic messages for exceptions

- `srange(range_spec)` - function to define a string of characters, given a string of the form used by regexp string ranges, such as `"[0-9]"` for all numeric digits, `"[A-Z_]"` for uppercase characters plus underscore, and so on (note that `range_spec` does not include support for generic regular expressions, just string range specs)

- `trace_parse_action(fn)` - decorator function to debug parse actions. Lists each call, called arguments, and return value or exception

### Helper parse actions

- `remove_quotes` - removes the first and last characters of a quoted string; useful to remove the delimiting quotes from quoted strings

- `replace_with(repl_string)` - returns a parse action that simply returns the `repl_string`; useful when using `transform_string`, or converting HTML entities, as in:

      nbsp = Literal("&nbsp;").set_parse_action(replace_with("<BLANK>"))

- `original_text_for`- restores any internal whitespace or suppressed text within the tokens for a matched parse expression. This is especially useful when defining expressions for `scan_string` or `transform_string` applications.

- `with_attribute(*args, **kwargs)` - helper to create a validating parse action to be used with start tags created with `make_xml_tags` or `make_html_tags`. Use `with_attribute` to qualify a starting tag with a required attribute value, to avoid false matches on common tags such as `<TD>` or `<DIV>`.

  `with_attribute` can be called with:

  - keyword arguments, as in `(class="Customer", align="right")`, or

  - a list of name-value tuples, as in `(("ns1:class", "Customer"), ("ns2:align", "right"))`

  An attribute can be specified to have the special value `with_attribute.ANY_VALUE`, which will match any value - use this to ensure that an attribute is present but any attribute value is acceptable.

- `match_only_at_col(column_number)` - a parse action that verifies that an expression was matched at a particular column, raising a `ParseException` if matching at a different column number; useful when parsing tabular data

- `common.convert_to_integer()` - converts all matched tokens to int

- `common.convert_to_float()` - converts all matched tokens to float

- `common.convert_to_date()` - converts matched token to a datetime.date

- `common.convert_to_datetime()` - converts matched token to a datetime.datetime

- `common.strip_html_tags()` - removes HTML tags from matched token

- `common.downcase_tokens()` - converts all matched tokens to lowercase

- `common.upcase_tokens()` - converts all matched tokens to uppercase

### Common string and token constants

- `alphas` - same as `string.ascii_letters`

- `nums` - same as `string.digits`

- `alphanums` - a string containing `alphas + nums`

- `alphas8bit` - a string containing alphabetic 8-bit characters:

      ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþ

- `identchars` - a string containing characters that are valid as initial identifier characters:

      ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyzª
      µºÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ

- `identbodychars` - a string containing characters that are valid as identifier body characters (those following a valid leading identifier character as given in identchars):

      0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ_abcdefghijklmnopqrstuvwxyzª
      µ·ºÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖØÙÚÛÜÝÞßàáâãäåæçèéêëìíîïðñòóôõöøùúûüýþÿ

- `printables` - same as `string.printable`, minus the space (`' '`) character

- `empty` - a global `Empty()`; will always match

- `sgl_quoted_string` - a string of characters enclosed in ‘s; may include whitespace, but not newlines

- `dbl_quoted_string` - a string of characters enclosed in “s; may include whitespace, but not newlines

- `quoted_string` - `sgl_quoted_string | dbl_quoted_string`

- `python_quoted_string` - `quoted_string | multiline quoted string`

- `c_style_comment` - a comment block delimited by `'/*'` and `'*/'` sequences; can span multiple lines, but does not support nesting of comments

- `html_comment` - a comment block delimited by `'<!--'` and `'-->'` sequences; can span multiple lines, but does not support nesting of comments

- `comma_separated_list` - similar to DelimitedList, except that the list expressions can be any text value, or a quoted string; quoted strings can safely include commas without incorrectly breaking the string into two tokens

- `rest_of_line` - all remaining printable characters up to but not including the next newline

- `common.integer` - an integer with no leading sign; parsed token is converted to int

- `common.hex_integer` - a hexadecimal integer; parsed token is converted to int

- `common.signed_integer` - an integer with optional leading sign; parsed token is converted to int

- `common.fraction` - signed_integer ‘/’ signed_integer; parsed tokens are converted to float

- `common.mixed_integer` - signed_integer ‘-’ fraction; parsed tokens are converted to float

- `common.real` - real number; parsed tokens are converted to float

- `common.sci_real` - real number with optional scientific notation; parsed tokens are convert to float

- `common.number` - any numeric expression; parsed tokens are returned as converted by the matched expression

- `common.fnumber` - any numeric expression; parsed tokens are converted to float

- `common.ieee_float` - any floating-point literal (int, real number, infinity, or NaN), returned as float

- `common.identifier` - a programming identifier (follows Python’s syntax convention of leading alpha or “_”, followed by 0 or more alpha, num, or “_”)

- `common.ipv4_address` - IPv4 address

- `common.ipv6_address` - IPv6 address

- `common.mac_address` - MAC address (with “:”, “-”, or “.” delimiters)

- `common.iso8601_date` - date in `YYYY-MM-DD` format

- `common.iso8601_datetime` - datetime in `YYYY-MM-DDThh:mm:ss.s(Z|+-00:00)` format; trailing seconds, milliseconds, and timezone optional; accepts separating `'T'` or `' '`

- `common.url` - matches URL strings and returns a ParseResults with named fields like those returned by `urllib.parse.urlparse()`

### Unicode character sets for international parsing

Pyparsing includes the `unicode` namespace that contains definitions for `alphas`, `nums`, `alphanums`, `identchars`, `identbodychars`, and `printables` for character ranges besides 7- or 8-bit ASCII. You can access them using code like the following:

    import pyparsing as pp
    ppu = pp.unicode

    greek_word = pp.Word(ppu.Greek.alphas)
    greek_word[...].parse_string("Καλημέρα κόσμε")

The following language ranges are defined.

Unicode set | Alternate names | Description
`Arabic` | العربية |
`Chinese` | 中文 |
`CJK` |  | Union of Chinese, Japanese, and Korean sets
`Cyrillic` | кириллица |
`Devanagari` | देवनागरी |
`Greek` | Ελληνικά |
`Hangul` | Korean, 한국어 |
`Hebrew` | עִברִית |
`Japanese` | 日本語 | Union of Kanji, Katakana, and Hiragana sets
`Japanese.Hiragana` | ひらがな |
`Japanese.Kanji` | 漢字 |
`Japanese.Katakana` | カタカナ |
`Latin1` |  | All Unicode characters up to code point 0x7f (255)
`LatinA` |  | Unicode characters for code points 0x100-0x17f (256-383)
`LatinB` |  | Unicode characters for code points 0x180-0x24f (384-591)
`Thai` | ไทย |
`BasicMultilingualPlane` | BMP | All Unicode characters up to code point 0xffff (65535)

The base `unicode` class also includes definitions based on all Unicode code points up to `sys.maxunicode`. This set will include emojis, wingdings, and many other specialized and typographical variant characters.

## Generating Railroad Diagrams

Grammars are conventionally represented in what are called “railroad diagrams”, which allow you to visually follow the sequence of tokens in a grammar along lines which are a bit like train tracks. You might want to generate a railroad diagram for your grammar in order to better understand it yourself, or maybe to communicate it to others.

### Usage

To generate a railroad diagram in pyparsing, you first have to install pyparsing with the `diagrams` extra. To do this, just run `pip install pyparsing[diagrams]`, and make sure you add `pyparsing[diagrams]` to any `setup.py` or `requirements.txt` that specifies pyparsing as a dependency.

Create your parser as you normally would. Then call `create_diagram()`, passing the name of an output HTML file.:

    street_address = Word(nums).set_name("house_number") + Word(alphas)[1, ...].set_name("street_name")
    street_address.set_name("street_address")
    street_address.create_diagram("street_address_diagram.html")

This will result in the railroad diagram being written to `street_address_diagram.html`.

create_diagram takes the following arguments:

- `output_html` (str or file-like object) - output target for generated diagram HTML

- `vertical` (int) - threshold for formatting multiple alternatives vertically instead of horizontally (default=3)

- `show_results_names` - bool flag whether diagram should show annotations for defined results names

- `show_groups` - bool flag whether groups should be highlighted with an unlabeled surrounding box

- `show_hidden` - bool flag whether internal pyparsing elements that are normally omitted in diagrams should be shown (default=False)

- `embed` - bool flag whether generated HTML should omit <HEAD>, <BODY>, and <DOCTYPE> tags to embed the resulting HTML in an enclosing HTML source (such as PyScript HTML)

- `head` - str containing additional HTML to insert into the <HEAD> section of the generated code; can be used to insert custom CSS styling

- `body` - str containing additional HTML to insert at the beginning of the <BODY> section of the generated code

### Example

You can view an example railroad diagram generated from a pyparsing grammar for SQL SELECT statements (generated from examples/select_parser.py).

### Naming tip

Parser elements that are separately named will be broken out as their own sub-diagrams. As a short-cut alternative to going through and adding `.set_name()` calls on all your sub-expressions, you can use `autoname_elements()` after defining your complete grammar. For example:

    a = pp.Literal("a")
    b = pp.Literal("b").set_name("bbb")
    pp.autoname_elements()

a will get named “a”, while b will keep its name “bbb”.

### Customization

You can customize the resulting diagram in a few ways. To do so, run `pyparsing.diagrams.to_railroad` to convert your grammar into a form understood by the railroad-diagrams module, and then `pyparsing.diagrams.railroad_to_html` to convert that into an HTML document. For example:

    from pyparsing.diagram import to_railroad, railroad_to_html

    with open('output.html', 'w') as fp:
        railroad = to_railroad(my_grammar)
        fp.write(railroad_to_html(railroad))

This will result in the railroad diagram being written to `output.html`

You can then pass in additional keyword arguments to `pyparsing.diagrams.to_railroad`, which will be passed into the `Diagram()` constructor of the underlying library, as explained here.

In addition, you can edit global options in the underlying library, by editing constants:

    from pyparsing.diagram import to_railroad, railroad_to_html
    import railroad

    railroad.DIAGRAM_CLASS = "my-custom-class"
    my_railroad = to_railroad(my_grammar)

These options are documented here.

Finally, you can edit the HTML produced by `pyparsing.diagrams.railroad_to_html` by passing in certain keyword arguments that will be used in the HTML template. Currently, these are:

- `head`: A string containing HTML to use in the `<head>` tag. This might be a stylesheet or other metadata

- `body`: A string containing HTML to use in the `<body>` tag, above the actual diagram. This might consist of a heading, description, or JavaScript.

If you want to provide a custom stylesheet using the `head` keyword, you can make use of the following CSS classes:

- `railroad-group`: A group containing everything relating to a given element group (ie something with a heading)

- `railroad-heading`: The title for each group

- `railroad-svg`: A div containing only the diagram SVG for each group

- `railroad-description`: A div containing the group description (unused)