# mp4h - Macro Processor for HTML Documents

This documentation describes mp4h version 1.3.1.

## Introduction

The mp4h software is a macro-processor specifically designed to deal with HTML documents. It allows powerful programming constructs, with a syntax familiar to HTML authors.

This software is based on `Meta-HTML`, written by Brian J. Fox, Even if both syntaxes look similar, source code is completely different. Indeed, a subset of Meta-HTML was used as a part of a more complex program, WML (`Website Meta Language`) written by Ralf S. Engelschall and which I maintain since January 1999. For licensing reasons, it was hard to hack Meta-HTML and so I decided to write my own macro-processor.

Instead of rewriting it from scratch, I preferred using another macro-processor engine. I chose `GNU m4`, written by Rene Seindal, because of its numerous advantages : this software is stable, robust and very well documented. This version of mp4h is derived from GNU m4 version 1.4n, which is a development version.

The mp4h software is not an HTML editor; its unique goal is to provide an easy way to define its own macros inside HTML documents. There is no plan to add functionalities to automagically produce valid HTML documents, if you want to clean up your code or validate it, simply use a post-processor like `tidy`.

## Command line options

Optional arguments are enclosed within square brackets. All option synonyms have a similar syntax, so when a long option accepts an argument, short option do too.

Syntax call is

    mp4h [options] [filename [filename] ...]

Options are described below. If no filename is specified, or if its name is `-`, then characters are read on standard input.

### Operation modes

--help | display an help message and exit
--version | output mp4h version information and exit
-E --fatal-warnings | stop execution after first warning
-Q --quiet --silent | suppress some warnings for builtins
-S --safety-level=`NUMBER` | disable risky functions; 0 means no filtering, 1 disable `execute` and 2 disable this one too plus all filesystem related functions: `file-exists`, `real-path`, `get-file-properties`, `directory-contents` and `include`.

### Preprocessor features

-I --include=`DIRECTORY` | search this directory for includes and packages
-D --define=`NAME[=VALUE]` | set variable NAME to VALUE, or empty
-U --undefine=`COMMAND` | delete builtin COMMAND
-s --synclines | generate `#line NO "FILE"' lines

### Parser features

-c --caseless=`NUMBER` | set case sensitiveness according to the bits of `NUMBER`. A null bit means symbol is case sensitive, and bits are defined as followed: 0 for tags, 1 for variables and 2 for entities. Default value is 3, i.e. only entities are case sensitive.
-e --encoding=`NAME` | specify document encoding. Valid options are `8bit' (default) or `utf8'.
-X --expansion=`NUMBER` | set parser behaviour according to the bits of `NUMBER`

NUMBER is a combination of

1 | do not parse unknown tags
2 | unknown tags are assumed being simple
4 | trailing star in tag name do not make this tag simple
8 | an unmatched end tag closes all previous unmatched begin tags
16 | interpret backslashes as printf
32 | remove trailing slash in tag attributes
64 | do not remove trailing star in tag name
128 | do not remove leading star in tag name
256 | do not add a space before trailing slash in tag attributes
1024 | suppress warnings about bad nested tags
2048 | suppress warnings about missing trailing slash

In version 1.3.1, default value is 3114=2+8+32+1024+2048.

### Limits control

-H --hashsize=`PRIME` | set symbol lookup hash table size (default 509)
-L -nesting-limit=`NUMBER` | change artificial nesting limit (default 250)

### Debugging

-d --debug=`FLAGS` | set debug level (no FLAGS implies `aeq')
-t --trace=`NAME` | trace NAME when it will be defined
-l --arglength=`NUMBER` | restrict macro tracing size
-o --error-output=`FILE` | redirect debug and trace output

Flags are any of:

t | trace for all macro calls, not only debugging-on'ed
a | show actual arguments
e | show expansion
c | show before collect, after collect and after call
x | add a unique macro call id, useful with c flag
f | say current input file name
l | say current input line number
p | show results of path searches
m | show results of module operations
i | show changes in input files
V | shorthand for all of the above flags

## Description

The mp4h software is a macro-processor, which means that keywords are replaced by other text. This chapter describes all primitives. As mp4h has been specially designed for HTML documents, its syntax is very similar to HTML, with tags and attributes. One important feature has no equivalent in HTML: comments until end of line. All text following three colons is discarded until end of line, like

    ;;;  This is a comment

### Function Macros

Note: All examples in this documentation are processed through mp4h with expansion flags set to zero (see a description of possible expansion flags at the end of document), it is why simple tags contain a trailing slash. But mp4h can output plain HTML files with other expansion flags.

The definition of new tags is the most common task provided by mp4h. As with HTML, macro names are case insensitive, unless `-c` option is used to change this default behaviour. In this documentation, only lowercase letters are used. There are two kinds of tags: simple and complex. A simple tag has the following form:

    <name [attributes] />

whereas a complex tag looks like:

    <name [attributes]>
    body
    </name>

Since version 0.9.1, mp4h knows XHTML syntax too, so your input file may conform to HTML or XHTML syntax. In this manual, we adopt the latter, which is why simple tags have a trailing slash in attributes. If you want to produce HTML files with this input file, you may either choose an adequate `--expansion` flag or use a post-processor like `tidy`.

When a simple tag is defined by mp4h, it can be parsed even if the trailing slash is omitted, because mp4h knows that this tag is simple. But it is a good practice to always append a trailing slash to simple tags.

In macro descriptions below, a slash indicates a simple tag, and a V letter that attributes are read verbatim (without expansion) (see the chapter on macro expansion for further details).

define-tag

`name`
`[attributes=verbatim]`
`[endtag=required]`
`[whitespace=delete]`

This function lets you define your own tags. First argument is the command name. Replacement text is the function body.

Source

    <define-tag foo>bar</define-tag>
    <foo/>

Output

    bar

Even if spaces have usually few incidence on HTML syntax, it is important to note that

    <define-tag foo>bar</define-tag>

and

    <define-tag foo>
    bar
    </define-tag>

are not equivalent, the latter form contains two newlines that were not present in the former.

- `whitespace=delete`
  Some spaces are suppressed in replacement text, in particular any leading or trailing spaces, and newlines not enclosed within angle brackets.
- `endtag=required`
  Define a complex tag

  Source

      <define-tag foo>bar</define-tag>
      <foo/>

  Output

      bar

  Source

      <define-tag bar endtag=required>;;;
      body is: %body</define-tag>
      <bar>Here it is</bar>

  Output

      body is: Here it is

- `attributes=verbatim`
  By default attributes are expanded before text is replaced. If this attribute is used, attributes are inserted into replacement text without expansion.

  Source

      <define-tag foo>quux</define-tag>
      <define-tag bar attributes=verbatim endtag=required>
      Body: %Ubody
      Attributes: %Uattributes
      </define-tag>
      <bar txt="<foo/>">Here we go</bar>

  Output

      Body: Here we go
      Attributes: txt=<foo/>

provide-tag

`name`
`[attributes=verbatim]`
`[endtag=required]`
`[whitespace=delete]`

This command is similar to the previous one, except that no operation is performed if this command was already defined.

`S`

let

`new=old`

Copy a function. This command is useful to save a macro definition before redefining it.

Source

    <define-tag foo>one</define-tag>
    <let bar=foo />
    <define-tag foo>two</define-tag>
    <foo/><bar/>

Output

    twoone

`S`

undef

`name`

Delete a command definition.

Source

    <define-tag foo>one</define-tag>
    <undef foo />
    <foo/>

Output

    <foo />

set-hook

`name`
`[position=before|after]`
`[action=insert|append|replace]`

Add text to a predefined macro. This mechanism allows modifications of existing macros without having to worry about its type, whether it is complex or not.

Source

    <let foo=add />
    <set-hook foo position=before>
    Before</set-hook>
    <set-hook foo position=after>
    After</set-hook>
    <foo 1 2 3 4 />

Output

    Before10
    After

`S`

get-hook

`name`
`[position=before|after]`

Print current hooks of a macro.

Source

    Text inserted with position=before:<get-hook foo position=before />!
    Text inserted with position=after:<get-hook foo position=after />!

Output

    Text inserted with position=before:
    Before!
    Text inserted with position=after:
    After!

`S`

attributes-quote

`%attributes`

Like `%attributes`, except that `attr=value` pairs are printed with double quotes surrounding attribute values, and a leading space is added if some text is printed.

Source

    <define-tag foo>;;;
    %attributes
    <img<attributes-quote %attributes />/>
    </define-tag>
    <foo id="logo" src="logo.gif" name="Logo" alt="Our logo" />
    <foo/>

Output

    id=logo src=logo.gif name=Logo alt=Our logo
    <img id="logo" src="logo.gif" name="Logo" alt="Our logo"/>

    <img/>

`S`

attributes-extract

`name1`
`[,name2[,name3...]]`
`%attributes`

Extract from `%attributes` the `attr=value` pairs for names matching any of name1, name2....

Source

    <define-tag img whitespace=delete>
    <img* <attributes-extract name,src,alt %attributes />  />
    </define-tag>
    <img id="logo" src="logo.gif" name="Logo" alt="Our logo"  />

Output

    <img src=logo.gif name=Logo alt=Our logo    />

`S`

attributes-remove

`name1`
`[,name2[,name3...]]`
`%attributes`

Remove from `%attributes` the `attr=value` pairs for names matching any of name1, name2....

Source

    <define-tag img whitespace=delete>
    <img* <attributes-quote <attributes-remove name,src,alt %attributes />/> />
    </define-tag>
    <img id="logo" src="logo.gif" name="Logo" alt="Our logo"  />

Output

    <img  id="logo"   />

Note: The two previous functions are special, because unlike all other macros, their expansion do not form a group. This is necessary to parse the resulting list of attributes.

In those two functions, names of attributes may be regular expressions. Main goal of these primitives is to help writing macros accepting any kind of attributes without having to declare them. A canonical example is

Source

    <define-tag href whitespace=delete>
    <preserve url name />
    <set-var <attributes-extract url,name %attributes />/>
    <a <attributes-quote <attributes-remove url,name %attributes />/>
       href="<get-var url />"><get-var name /></a>
    <restore  url name />
    </define-tag>
    <href class=web url="http://www.foo.com" name="Welcome"  />

Output

    <a  class="web"
       href="http://www.foo.com">Welcome</a>

But we want now to add an image attribute. So we may write

Source

    <define-tag href whitespace=delete>
    <preserve url name image />
    <set-var <attributes-extract url,name,image %attributes />/>
    <a <attributes-quote <attributes-remove url,name,image %attributes />/>
       href="<get-var url />">
    <if <get-var image />
       <img <attributes-quote <attributes-remove url,name,image %attributes />/>
          src="<get-var image />" alt="<get-var name />" border=0  />
      <get-var name />
    />
    </a>
    <restore  url name image />
    </define-tag>
    <href class=web url="http://www.foo.com" name="Welcome" image="foo.png"/>

Output

    <a  class="web"
       href="http://www.foo.com"><img  class="web"
          src="foo.png" alt="Welcome" border=0      /></a>

We need a mechanism to tell mp4h that some attributes refer to specific HTML tags. A solution is to prepend attribute with tag name, e.g.

Source

    <define-tag href whitespace=delete>
    <preserve url name image />
    <set-var <attributes-extract url,name,image %attributes />/>
    <a <attributes-quote <attributes-extract a:.* %attributes />/>
       href="<get-var url />">
    <if <get-var image />
       <img <attributes-quote <attributes-extract img:.* %attributes />/>
          src="<get-var image />" alt="<get-var name />"  />
      <get-var name />
    />
    </a>
    <restore  url name image />
    </define-tag>
    <href a:class=web img:id=logo img:border=1
          url="http://www.foo.com" name="Welcome" image="foo.png" />

Output

    <a  a:class="web"
       href="http://www.foo.com"><img  img:id="logo" img:border="1"
          src="foo.png" alt="Welcome"      /></a>

This example shows that regular expressions may be used within attributes names, but it is still incomplete, because we want to remove prefix from attributes. One solution is with `subst-in-string`, but there is a more elegant one:

Source

    <define-tag href whitespace=delete>
    <preserve url name image />
    <set-var <attributes-extract url,name,image %attributes />/>
    <a <attributes-quote <attributes-extract :a:(.*) %attributes />/>
       href="<get-var url />">
    <if <get-var image />
       <img <attributes-quote <attributes-extract :img:(.*) %attributes />/>
          src="<get-var image />" alt="<get-var name />"  />
      <get-var name />
    />
    </a>
    <restore  url name image />
    </define-tag>
    <href :a:class=web :img:id=logo :img:border=1
          url="http://www.foo.com" name="Welcome" image="foo.png" />

Output

    <a  class="web"
       href="http://www.foo.com"><img  id="logo" border="1"
          src="foo.png" alt="Welcome"      /></a>

When there are subexpressions within regular expressions, they are printed instead of the whole expression. Note also that I put a colon before the prefix in order not to mix them with XML namespaces.

### Entities

Entities are macros in the same way as tags, but they do not take any arguments. Whereas tags are normally used to mark up text, entities contain already marked up text. Also note that unlike tags, entities are by default case sensitive.

An entity has the following form:

    &entity;

define-entity

`name`

This function lets you define your own entities. First argument is the entity name. Replacement text is the function body.

Source

    <define-entity foo>bar</define-entity>
    &foo;

Output

    bar

### Variables

Variables are a special case of simple tags, because they do not accept attributes. In fact their use is different, because variables contain text whereas macros act like operators. A nice feature concerning variables is their manipulation as arrays. Indeed variables can be considered like newline separated lists, which will allow powerful manipulation functions as we will see below.

`S`

set-var

`name[=value]`
`[name[=value]] ...`

This command sets variables.

`S`

`V`

set-var-verbatim

`name[=value]`
`[name[=value]] ...`

As above but attributes are read verbatim.

set-var-x

`name=variable-name`

This command assigns a variable the value of the body of the command. This is particularly useful when variable values contain newlines and/or quotes.

Note that the variable can not be indexed with this command. Note also, that this command behaves as set-var-verbatim: The body is not expanded until the variable is shown with get-var.

`S`

get-var

`name`
`[name] ...`

Show variable contents. If a numeric value within square brackets is appended to a variable name, it represents the index of an array. The first index of arrays is 0 by convention.

Source

    <set-var version="0.10.1" />
    This is version <get-var version />
    <set-var-x name="osversion">Operating system is
    "<include command="uname" />"</set-var-x>
    <get-var osversion />

Output

    This is version 0.10.1

    Operating system is
    "Linux
    "

Source

    <set-var foo="0
    1
    2
    3" />
    <get-var foo[2] foo[0] foo />

Output

    200
    1
    2
    3

`S`

`V`

get-var-once

`name`
`[name] ...`

As above but attributes are not expanded.

Source

    <define-tag foo>0.10.1</define-tag>
    <set-var version="<foo/>" />;;;
    Here is version <get-var version />
    <set-var-verbatim version="<foo/>" />;;;
    Here is version <get-var version />
    <set-var-verbatim version="<foo/>" />;;;
    Here is version <get-var-once version />

Output

    Here is version 0.10.1
    Here is version 0.10.1
    Here is version <foo/>

`S`

preserve

`name`
`[name] ...`

All variables are global, there is no variable or macro scope. For this reason a stack is used to preserve variables. When this command is invoked, arguments are names of variables, whose values are put at the top of the stack and variables are reset to an empty string.

`S`

restore

`name`
`[name] ...`

This is the opposite: arguments are names of variables, which are set to the value found at the top of the stack, and stack is popped down.

Note: The `preserve` tag pushes its last argument first, whereas `restore` first pops its first argument.

Source

    <define-tag foo whitespace=delete>
    <preserve src name text />
    <set-var %attributes />
    Inside: src=<get-var src /> name=<get-var name /> text=<get-var text />
    <restore  src name text />
    </define-tag>
    <set-var src=foo.png text="Hello, World!" />
    Before: src=<get-var src /> name=<get-var name /> text=<get-var text />
    <foo src=bar name=quux />
    After: src=<get-var src /> name=<get-var name /> text=<get-var text />

Output

    Before: src=foo.png name= text=Hello, World!
    Inside: src=bar name=quux text=
    After: src=foo.png name= text=Hello, World!

`S`

unset-var

`name`
`[name] ...`

Undefine variables.

`S`

var-exists

`name`

Returns `true` when this variable exists.

`S`

increment

`name`
`[by=value]`

Increment the variable whose name is the first argument. Default increment is one.

- `by=value`
  Change increment amount.

Source

    <set-var i=10 />
    <get-var i />
    <increment i /><get-var i />
    <increment i by="-3" /><get-var i />

Output

    10
    11
    8

`S`

decrement

`name`
`[by=value]`

Decrement the variable whose name is the first argument. Default decrement is one.

- `by=value`
  Change decrement amount.

Source

    <set-var i=10 />
    <get-var i />
    <decrement i /><get-var i />
    <decrement i by="3" /><get-var i />

Output

    10
    9
    6

`S`

copy-var

`src`
`dest`

Copy a variable into another.

Source

    <set-var i=10 />
    <copy-var i j />
    <get-var j />

Output

    10

`S`

defvar

`name`
`value`

If this variable is not defined or is defined to an empty string, then it is set to the second argument.

Source

    <unset-var title />
    <defvar title "Title" /><get-var title />
    <defvar title "New title" /><get-var title />

Output

    Title
    Title

`S`

symbol-info

`name`

Show information on symbols. If it is a variable name, the `STRING` word is printed as well as the number of lines contained within this variable. If it is a macro name, one of the following messages is printed: `PRIM COMPLEX`, `PRIM TAG`, `USER COMPLEX` or `USER TAG`

Source

    <set-var x="0\n1\n2\n3\n4" />
    <define-tag foo>bar</define-tag>
    <define-tag bar endtag=required>quux</define-tag>
    <symbol-info x />
    <symbol-info symbol-info />
    <symbol-info define-tag />
    <symbol-info foo />
    <symbol-info bar />

Output

    STRING
    5
    PRIM TAG
    PRIM COMPLEX
    USER TAG
    USER COMPLEX

### String Functions

`S`

string-length

`string`

Prints the length of the string.

Source

    <set-var foo="0
    1
    2
    3" />;;;
    <string-length <get-var foo /> />
    <set-var foo="0 1 2 3" />;;;
    <set-var l=<string-length <get-var foo /> /> />;;;
    <get-var l />

Output

    7
    7

`S`

downcase

`string`

Convert to lowercase letters.

Source

    <downcase "Does it work?" />

Output

    does it work?

`S`

upcase

`string`

Convert to uppercase letters.

Source

    <upcase "Does it work?" />

Output

    DOES IT WORK?

`S`

capitalize

`string`

Convert to a title, with a capital letter at the beginning of every word.

Source

    <capitalize "Does it work?" />

Output

    Does It Work?

`S`

substring

`string`
`[start [end]]`

Extracts a substring from a string. First argument is original string, second and third are respectively start and end indexes. By convention first character has a null index.

Source

    <set-var foo="abcdefghijk" />
    <substring <get-var foo /> 4 />
    <substring <get-var foo /> 4 6 />

Output

    efghijk
    ef

`S`

string-eq

`string1`
`string2`
`[caseless=true]`

Returns `true` if first two arguments are equal.

Source

    1:<string-eq "aAbBcC" "aabbcc" />
    2:<string-eq "aAbBcC" "aAbBcC" />

Output

    1:
    2:true

- `caseless=true`
  Comparison is case insensitive.

Source

    1:<string-eq "aAbBcC" "aabbcc" caseless=true />
    2:<string-eq "aAbBcC" "aAbBcC" caseless=true />

Output

    1:true
    2:true

`S`

string-neq

`string1`
`string2`
`[caseless=true]`

Returns `true` if the first two arguments are not equal.

Source

    1:<string-neq "aAbBcC" "aabbcc" />
    2:<string-neq "aAbBcC" "aAbBcC" />

Output

    1:true
    2:

- `caseless=true`
  Comparison is case insensitive.

Source

    1:<string-neq "aAbBcC" "aabbcc" caseless=true />
    2:<string-neq "aAbBcC" "aAbBcC" caseless=true />

Output

    1:
    2:

`S`

string-compare

`string1`
`string2`
`[caseless=true]`

Compares two strings and returns one of the values less, greater or equal depending on this comparison.

Source

    1:<string-compare "aAbBcC" "aabbcc" />
    2:<string-compare "aAbBcC" "aAbBcC" />

Output

    1:less
    2:equal

- `caseless=true`
  Comparison is case insensitive.

Source

    1:<string-compare "aAbBcC" "aabbcc" caseless=true />

Output

    1:equal

`S`

char-offsets

`string`
`character`
`[caseless=true]`

Prints an array containing indexes where the character appear in the string.

- `caseless=true`
  Comparison is case insensitive.

Source

    1:<char-offsets "abcdAbCdaBcD" a />
    2:<char-offsets "abcdAbCdaBcD" a caseless=true />

Output

    1:0
    8
    2:0
    4
    8

`S`

printf

`format`
`string`
`[string ...]`

Prints according to a given format. Currently only the `%s` flag character is recognized, and `$` extension is supported to change order of arguments.

Source

    1:<printf "foo %s bar %s" baz 10 />
    2:<printf "foo %2$s bar %1$s" baz 10 />

Output

    1:foo baz bar 10
    2:foo 10 bar baz

### Regular Expressions

Regular expression support is provided by the PCRE (Perl Compatible Regular Expressions) library package, which is open source software, copyright by the University of Cambridge. This is a very nice piece of software, latest versions are available at `ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre/`.

Before version 1.0.6, POSIX regular expressions were implemented. For this reason, the following macros recognize two attributes, `caseless=true` and `singleline=true|false`. But Perl allows a much better control on regular expressions with so called modifiers, which are assed to the new `reflags` attribute. It may contain one or more modifiers:

i | Matching is case insensitive
m | Treat string as multiple lines. When set, a `^` matches any beginning of line, and `$` any end of line. By default, they match begin and end of string.
s | Treat string as single line. A dot (`.`) may also match a newline, whereas it does not by default.
x | Allow formatted regular expression, that means whitespaces, newlines and comments are removed from regular expression before processing.

Note: Attribute `singleline=true` is a synonym for the `s` modifier, whereas `singleline=false` is a synonym for the `m` modifier. This behaviour was different up to mp4h 1.0.6.

`S`

subst-in-string

`string`
`regexp`
`[replacement]`
`[caseless=true]`
`[singleline=true|false]`
`[reflags=[imsx]]`

Replace a regular expression in a string by a replacement text.

Source

    <set-var foo="abcdefghijk" />
    <subst-in-string <get-var foo /> "[c-e]" />
    <subst-in-string <get-var foo /> "([c-e])" "\\1 " />

Output

    abfghijk
    abc d e fghijk

Source

    <set-var foo="abcdefghijk\nabcdefghijk\nabcdefghijk" />
    <subst-in-string <get-var foo /> ".$" "" />
    <subst-in-string <get-var foo /> ".$" "" singleline=false />
    <subst-in-string <get-var foo /> "
       ([a-c]) | [0-9]
         " ":\\1:" reflags=x />

Output

    abcdefghijk
    abcdefghijk
    abcdefghij
    abcdefghij
    abcdefghij
    abcdefghij
    :a::b::c:defghijk
    :a::b::c:defghijk
    :a::b::c:defghijk

`S`

subst-in-var

`name`
`regexp`
`[replacement]`
`[caseless=true]`
`[singleline=true|false]`
`[reflags=[imsx]]`

Performs substitutions inside variable content.

`S`

match

`string`
`regexp`
`[caseless=true]`
`[singleline=true|false]`
`[reflags=[imsx]]`
`[action=report|extract|delete|startpos|endpos|length]`

- `action=report`
  Prints `true` if string contains regexp.
- `action=extract`
  Prints the expression matching regexp in string.
- `action=delete`
  Prints the string without the expression matching regexp in string.
- `action=startpos`
  Prints the first char of the expression matching regexp in string. If there is no match, returns `-1`.
- `action=endpos`
  Prints the last char of the expression matching regexp in string. If there is no match, returns `-1`.
- `action=length`
  Prints the length of the expression matching regexp in string.

Source

    1:<match "abcdefghijk" "[c-e]+" />
    2:<match "abcdefghijk" "[c-e]+" action=extract />
    3:<match "abcdefghijk" "[c-e]+" action=delete />
    4:<match "abcdefghijk" "[c-e]+" action=startpos />
    5:<match "abcdefghijk" "[c-e]+" action=endpos />
    6:<match "abcdefghijk" "[c-e]+" action=length />

Output

    1:true
    2:cde
    3:abfghijk
    4:2
    5:5
    6:3

### Arrays

With mp4h one can easily deal with string arrays. Variables can be treated as a single value or as a newline separated list of strings. Thus after defining

    <set-var digits="0
    1
    2
    3" />

one can view its content or one of these values:

Source

    <get-var digits />
    <get-var digits[2] />

Output

    0
    1
    2
    3
    2

`S`

array-size

`name`

Returns an array size which is the number of lines present in the variable.

Source

    <array-size digits />

Output

    4

`S`

array-push

`name`
`value`

Add a value (or more if this value contains newlines) at the end of an array.

Source

    <array-push digits "10\n11\n12" />
    <get-var digits />

Output

    0
    1
    2
    3
    10
    11
    12

`S`

array-pop

`name`

Remove the toplevel value of an array and returns this string.

`S`

array-topvalue

`name`

Prints the last entry of an array.

Source

    <array-topvalue digits />

Output

    12

`S`

array-add-unique

`name`
`value`
`[caseless=true]`

Add a value at the end of an array if this value is not already present in this variable.

Source

    <array-add-unique digits 2 />
    <get-var digits />

Output

    0
    1
    2
    3
    10
    11
    12

- `caseless=true`
  Comparison is case insensitive.

`S`

array-concat

`name1`
`[name2] ...`

Concatenates all arrays into the first one.

Source

    <set-var foo="foo" />
    <set-var bar="bar" />
    <array-concat foo bar /><get-var foo />

Output

    foo
    bar

`S`

array-member

`name`
`value`
`[caseless=true]`

If value is contained in array, returns its index otherwise returns -1.

Source

    <array-member digits 11 />

Output

    5

- `caseless=true`
  Comparison is case insensitive.

`S`

array-shift

`name`
`offset`
`[start=start]`

Shifts an array. If offset is negative, indexes below 0 are lost. If offset is positive, first indexes are filled with empty strings.

Source

    <array-shift digits 2 />
    Now: <get-var digits />
    <array-shift digits -4 />
    And: <get-var digits />

Output

    Now:

    0
    1
    2
    3
    10
    11
    12

    And: 2
    3
    10
    11
    12

- `start=start`
  Change origin of shifts (default is 0).

  Source

      <array-shift digits -2 start=2 /><get-var digits />

  Output

      2
      3
      12

`S`

sort

`name`
`[caseless=true]`
`[numeric=true]`
`[sortorder=reverse]`

Sort lines of an array in place. Default is to sort lines alphabetically.

Source

    <sort digits /><get-var digits />

Output

    12
    2
    3

- `caseless=true`
  Comparison is case insensitive.
- `numeric=true`
  Sort lines numerically

  Source

      <sort digits numeric=true /><get-var digits />

  Output

      2
      3
      12

- `sortorder=reverse`
  Reverse sort order

  Source

      <sort digits numeric=true sortorder=reverse />;;;
      <get-var digits />

  Output

      12
      3
      2

### Numerical operators

These operators perform basic arithmetic operations. When all operands are integers result is an integer too, otherwise it is a float. These operators are self-explanatory.

`S`

add

`number1`
`number2`
`[number3] ...`

`S`

substract

`number1`
`number2`
`[number3] ...`

`S`

multiply

`number1`
`number2`
`[number3] ...`

`S`

divide

`number1`
`number2`
`[number3] ...`

`S`

min

`number1`
`number2`
`[number3] ...`

`S`

max

`number1`
`number2`
`[number3] ...`

Source

    <add 1 2 3 4 5 6 />
    <add 1 2 3 4 5 6. />

Output

    21
    21.000000

Source

    <define-tag factorial whitespace=delete>
    <ifeq %0 1 1 <multiply %0 "<factorial <substract %0 1 /> />" /> />
    </define-tag>
    <factorial 6 />

Output

    720

`S`

modulo

`number1`
`number2`

Unlike functions listed above the modulo function cannot handle more than 2 arguments, and these arguments must be integers.

Source

    <modulo 345 7 />

Output

    2

Those functions compare two numbers and returns `true` when this comparison is true. If one argument is not a number, comparison is false.

`S`

gt

`number1`
`number2`

Returns `true` if first argument is greater than second.

`S`

lt

`number1`
`number2`

Returns `true` if first argument is lower than second.

`S`

eq

`number1`
`number2`

Returns `true` if arguments are equal.

`S`

neq

`number1`
`number2`

Returns `true` if arguments are not equal.

### Relational operators

`S`

not

`string`

Returns `true` if string is empty, otherwise returns an empty string.

`S`

and

`string`
`[string] ...`

Returns the last argument if all arguments are non empty.

`S`

or

`string`
`[string] ...`

Returns the first non empty argument.

### Flow functions

`S`

`V`

group

`expression`
`[expression] ...`
`[separator=string]`

This function groups multiple statements into a single one. Some examples will be seen below with conditional operations.

A less intuitive but very helpful use of this macro is to preserve newlines when `whitespace=delete` is specified.

Source

    <define-tag text1>
    Text on
    3 lines without
    whitespace=delete
    </define-tag>
    <define-tag text2 whitespace=delete>
    Text on
    3 lines with
    whitespace=delete
    </define-tag>
    <define-tag text3 whitespace=delete>
    <group "Text on
    3 lines with
    whitespace=delete" />
    </define-tag>
    <text1/>
    <text2/>
    <text3/>

Output

    Text on
    3 lines without
    whitespace=delete

    Text on3 lines withwhitespace=delete
    Text on
    3 lines with
    whitespace=delete

Note that newlines are suppressed in `text2` and result is certainly unwanted.

compound

`expression`
`[expression] ...`
`[separator=string]`

Like `group`, but this tag is complex.

- `separator=string`
  By default arguments are put aside. This attribute define a separator inserted between arguments.

`S`

disjoin

`expression`

Does the opposite job to `group`, its argument is no more treated as a single object when processed by another command.

`S`

`V`

noexpand

`command`
`[command] ...`

Prints its arguments without expansion. They will never be expanded unless the `expand` tag is used to cancel this `noexpand` tag.

`S`

expand

`command`
`[command] ...`

Cancels the `noexpand` tag.

Source

    <subst-in-string "=LT=define-tag foo>bar=LT=/define-tag>" "=LT=" "<" />
    <foo/>
    <subst-in-string "=LT=define-tag foo>quux=LT=/define-tag>" "=LT="
       "<noexpand "<" />" />
    <foo/>

Output

    bar
    <define-tag foo>quux</define-tag>
    bar

`S`

`V`

if

`string`
`then-clause`
`[else-clause]`

If string is non empty, second argument is evaluated otherwise third argument is evaluated.

Source

    <define-tag test whitespace=delete>
    <if %0 "yes" "no" />
    </define-tag>
    <test "string" />
    <test "" />

Output

    yes
    no

`S`

`V`

ifeq

`string1`
`string2`
`then-clause`
`[else-clause]`

If first two arguments are identical strings, third argument is evaluated otherwise fourth argument is evaluated.

`S`

`V`

ifneq

`string1`
`string2`
`then-clause`
`[else-clause]`

If first two arguments are not identical strings, third argument is evaluated otherwise fourth argument is evaluated.

when

`string`

When argument is not empty, its body is evaluated.

`V`

while

`cond`

While condition is true, body function is evaluated.

Source

    <set-var i=10 />
    <while <gt <get-var i /> 0 />>;;;
      <get-var i /> <decrement i />;;;
    </while>

Output

    10 9 8 7 6 5 4 3 2 1

foreach

`variable`
`array`
`[start=start]`
`[end=end]`
`[step=pas]`

This macro is similar to the `foreach` Perl's macro: a variable loops over array values and function body is evaluated for each value. first argument is a generic variable name, and second is the name of an array.

Source

    <set-var x="1\n2\n3\n4\n5\n6" />
    <foreach i x><get-var i /> </foreach>

Output

    1 2 3 4 5 6

- `start=start`
  Skips first indexes.

  Source

      <set-var x="1\n2\n3\n4\n5\n6" />
      <foreach i x start=3><get-var i /> </foreach>

  Output

      4 5 6

- `end=end`
  Stops after index has reached that value.

  Source

      <set-var x="1\n2\n3\n4\n5\n6" />
      <foreach i x end=3><get-var i /> </foreach>

  Output

      1 2 3

- `step=step`
  Change index increment (default is 1). If step is negative, array is treated in reverse order.

  Source

      <set-var x="1\n2\n3\n4\n5\n6" />
      <foreach i x step=2><get-var i /> </foreach>
      <foreach i x step=-2><get-var i /> </foreach>

  Output

      1 3 5
      6 4 2

`S`

`V`

var-case

`var1=value1 action1`
`[var2=value2 action2 ...`

This command performs multiple conditions with a single instruction.

Source

    <set-var i=0 />
    <define-tag test>
    <var-case
      x=1   <group <increment i /> x<get-var i /> />
      x=2   <group <decrement i /> x<get-var i /> />
      y=1   <group <increment i /> y<get-var i /> />
      y=2   <group <decrement i /> y<get-var i /> />
    />
    </define-tag>
    <set-var x=1 y=2 /><test/>
    <set-var x=0 y=2 /><test/>

Output

    x1y0

    y-1

`S` |  | break |

Breaks the innermost `while` loop.

Source

    <set-var i=10 />
    <while <gt <get-var i /> 0 />>;;;
      <get-var i /> <decrement i />;;;
      <ifeq <get-var i /> 5 <break/> />;;;
    </while>

Output

    10 9 8 7 6

`S`

return

`[up=number]`
`string`

This command immediately exits from the innermost macro. A message may also be inserted. But this macro changes token parsing so its use may become very hazardous in some situations.

- `up=number`
  This attribute determines how much levels have to be exited. By default only one level is skipped. With a null value, all current macros are exited from. A negative value do the same, and stops processing current file.

`S`

warning

`string`

Prints a warning on standard error.

`S`

exit

`[status=rc]`
`[message=string]`

Immediately exits program.

- `message=string`
  Prints a message to the standard error.
- `status=rc`
  Selects the code returned by the program (-1 by default).

|  | at-end-of-file |

This is a special command: its content is stored and will be expanded after end of input.

### File functions

`S`

directory-contents

`dirname`
`[matching=regexp]`

Returns a newline separated list of files contained in a given directory.

Source

    <directory-contents . matching=".*\\.mp4h$" />

Output

    mp4h.mp4h

`S`

real-path

`patname=pathname`

Resolves all symbolic links, extra ``/'' characters and references to /./ and /../ in pathname, and expands into the resulting absolute pathname. All but the last component of pathname must exist when real-path is called.

This tag is particularly useful when comparing if file or directory names are identical.

Source

    <real-path pathname=<__file__/> />

Output

    /build/mp4h-ztsoyv/mp4h-1.3.1/doc/mp4h.mp4h

`S`

file-exists

`filename`

Returns `true` if file exists.

`S`

get-file-properties

`filename`

Returns an array of information on this file. These information are: size, type, ctime, mtime, atime, owner and group.

Source

    <get-file-properties <__file__/> />

Output

    68604
    FILE
    1767645040
    1767645040
    1767645040
    root
    root

`S`

include

`file=filename | command=command-line`
`[alt=action]`
`[verbatim=true]`

Insert the contents of a file in the file system - if the `file` attribute is given -, or the output from executing a system command - if the `command` attribute is given - into the input stream. For backwards compatibility, if neither the `file` nor the `command` attributes are given, the first argument is taken as a file to include.

- `file=filename`

  The given file is read and inserted into the input stream. This attribute cannot be combined with the command attribute.

  Files are first searched in current directory, then in directories specified on command-line with the `-I` option, next in directories listed in the `MP4HLIB` environment variable (it used to be `MP4HPATH` for versions prior to 1.3), and last under the compile-time location (`/usr/local/lib/mp4h/1.3.1:/usr/local/share/mp4h` by default).

- `command=command-line`

  The given command line is executed on the operating system, and the output of it is inserted in the input stream. This attribute cannot be combined with the file attribute.

  The given command line is executed using the popen(3) standard C library routine. The command is executed using the standard system shell, which on Posix compliant systems is sh(1).

- `alt=action`
  If file is not found, this alternate action is handled. If this attribute is not set and file is not found, then an error is raised. This attribute has no effect when the command attribute is specified.
- `verbatim=true`
  File content is included without expansion. This is similar to using the m4 undivert macro with a filename as argument.

Source

    <include command="uname -s" />

Output

    Linux

`S`

use

`name=package`

Load definitions from a package file.

|  | comment |

This tag does nothing, its body is simply discarded.

`S`

set-eol-comment

`[string]`

Change comment characters.

`S`

set-quotes

`[string string]`
`[display=visible]`

By default, all characters between `<@[` and `]@>` pairs are read without parsing. When called without argument, this macro inhibates this feature. When called with two arguments, it redefines begin and end delimiters. Begin delimiter must begin with a left-angle bracket, and end delimiter must end with a right-angle bracket.

- `display=visible`
  Delimiters are also written into output.

### Diversion functions

Diversions are a way of temporarily saving output. The output of mp4h can at any time be diverted to a temporary file, and be reinserted into the output stream, undiverted, again at a later time.

Numbered diversions are counted from 0 upwards, diversion number 0 being the normal output stream. The number of simultaneous diversions is limited mainly by the memory used to describe them, because mp4h tries to keep diversions in memory. However, there is a limit to the overall memory usable by all diversions taken altogether. When this maximum is about to be exceeded, a temporary file is opened to receive the contents of the biggest diversion still in memory, freeing this memory for other diversions. So, it is theoretically possible that the number of diversions be limited by the number of available file descriptors.

`S`

divert

`[ divnum=diversion-number ]`

Output is diverted using this tag, where diversion-number is the diversion to be used. If the divnum attribute is left out, diversion-number is assumed to be zero. If output is diverted to a non-existent diversion, it is simply discarded. This can be used to suppress unwanted output. See the example below.

When all mp4h input will have been processed, all existing diversions are automatically undiverted, in numerical order.

Several calls of divert with the same argument do not overwrite the previous diverted text, but append to it.

Source

    <divert divnum="-1"/>
    This is sent nowhere...
    <divert/>
    This is output.

Output

    This is sent nowhere...

    This is output.

`S`

undivert

`[ divnum=diversion-number ]`

This tag explicitly undiverts diverted text saved in the diversion with the specified number. If the divnum attribute is not given, all diversions are undiverted, in numerical order.

When diverted text is undiverted, it is not reread by mp4h, but rather copied directly to the current output. It is therefore not an error to undivert into a diversion.

Unlike m4, the mp4h undivert tag does not allow a file name as argument. The same can be accomplished with the include tag with the verbatim="true" attribute.

Source

    <divert divnum="1"/>
    This text is diverted.
    <divert/>
    This text is not diverted.
    <undivert divnum="1"/>

Output

    This text is diverted.

    This text is not diverted.

`S` |  | divnum |

This tag expands to the number of the current diversion.

Source

    Initial <divnum/>
    <divert divnum="1"/>
    Diversion one: <divnum/>
    <divert divnum="2"/>
    Diversion two: <divnum/>
    <divert/>

Output

    Initial 0

    Diversion one: 1

    Diversion two: 2

### Debugging functions

When constructs become complex it could be hard to debug them. Functions listed below are very useful when you could not figure what is wrong. These functions are not perfect yet and must be improved in future releases.

`S`

function-def

`name`

Prints the replacement text of a user defined macro. For instance, the macro used to generate all examples of this documentation is

Source

    <function-def example />

Output

    <set-var-verbatim verb-body=%ubody />
    <subst-in-var verb-body "&" "&amp;" />
    <subst-in-var verb-body "<" "&lt;" />
    <subst-in-var verb-body ">" "&gt;" />
    <set-var body=%body />
    <subst-in-var body "<(/?[a-z]*)[*]" "<\\1" />
    <subst-in-var body "&" "&amp;" />
    <subst-in-var body "<" "&lt;" />
    <subst-in-var body ">" "&gt;" />
    <subst-in-var body "<three-colon/>[^;\n]*\n[ \t]*" "" />
    <subst-in-var body "<three-colon/>$" "" reflags=m />
    <subst-in-var body "^\n*(.*?)\n*$" "\\1" reflags=s />
    <table border=2 cellpadding=0 cellspacing=0 width="80%" summary="">
        <tr><th bgcolor="#ccccff" align=left>&nbsp;&nbsp;&nbsp;Source</th></tr>
        <tr><td bgcolor="#ccff99" width="80%"><dnl/>
          <pre><get-var-once verb-body /></pre><dnl/>
        </td></tr>
        <tr><th bgcolor="#ccccff" align=left>&nbsp;&nbsp;&nbsp;Output</th></tr>
        <tr><td bgcolor="#ff99cc" width="80%"><dnl/>
          <pre><get-var-once body /></pre><dnl/>
        </td></tr>
    </table>

`S`

debugmode

`string`

This command acts like the `-d` flag but can be ynamically changed.

`S`

debugfile

`filename`

Selects a file where debugging messages are diverted. If this filename is empty, debugging messages are sent back to standard error, and if it is set to `-` these messages are discarded.

Note: There is no way to print these debugging messages into the document being processed.

`S`

debugging-on

`name`
`[name] ...`

Declare these macros traced, i.e. information about these macros will be printed if `-d` flag or `debugmode` macro are used.

`S`

debugging-off

`name`
`[name] ...`

These macros are no more traced.

### Miscellaneous

`S`

__file__

`[name]`

Without argument this macro prints current input filename. With an argument, this macro sets the string returned by future invocation of this macro.

`S`

__line__

`[number]`

Without argument this macro prints current number line in input file. With an argument, this macro sets the number returned by future invocation of this macro.

Source

    This is <__file__/>, line <__line__/>.

Output

    This is ./mp4h.mp4h, line 2201.

If you closely look at source code you will see that this number is wrong. Indeed the number line is the end of the entire block containing this instruction.

`S` |  | __version__ |

Prints the version of mp4h.

`S` |  | dnl |

Discard all characters until newline is reached. This macro ensures that following string is a comment and does not depend of the value of comment characters.

Source

    <dnl/>This is a comment
    foo
    <dnl/>This is a comment
    bar

Output

    foo
    bar

`S`

date

`[epoch]`

Prints local time according to the epoch passed on argument. If there is no argument, current local time is printed.

- `time`
  An epoch time specification.
- `format`
  A format specification as used with the strftime(3) C library routine.

Source

    <date/>
    <set-var info=<get-file-properties <__file__/> /> />
    <date <get-var info[2] /> />
    <date time="<get-var info[2] />" format="%Y-%m-%d %H:%M:%S" />

Output

    Mon Jan  5 20:30:40 2026

    Mon Jan  5 20:30:40 2026
    2026-01-05 20:30:40

`S` |  | timer |

Prints the time spent since last call to this macro. The printed value is the number of clock ticks, and so is dependent of your CPU.

Source

    <timer/>
    The number of clock ticks since the beginning of the parsing of
    this example by &mp4h; is:
    <timer/>

Output

    user 23
    sys 23

    The number of clock ticks since the beginning of the parsing of
    this example by <b>mp4h</b> is:
    user 23
    sys 23

`S`

mp4h-l10n

`name=value`

Set locale-specific variables. By default, the portable "C" locale is selected. As locales have different names on different platforms, you must refer to your system documentation to find which values are adapted to your system.

`S`

mp4h-output-radix

`number`

Change the output format of floats by setting the number of digits after the decimal point. Default is to print numbers in the "%6.f" format.

Source

    <add 1.2 3.4 />
    <mp4h-output-radix 2 />
    <add 1.2 3.4 />

Output

    4.600000

    4.60

## External packages

It is possible to include external files with the `include` command. Files are first searched in current directory, then in directories specified on command-line with the `-I` option, next in directories listed in the `MP4HLIB` environment variable (it used to be `MP4HPATH` for versions prior to 1.3), and last under the compile-time location (`/usr/local/lib/mp4h/1.3.1:/usr/local/share/mp4h` by default).

Another way to include packages is with the `use` command. There are two differences between `use` and `include`: first, package name has no suffix; and more important, a package cannot be loaded more than once.

## Macro expansion

This part describes internal mechanism of macro expansion. It must be as precise and exhaustive as possible so `contact me` if you have any suggestion.

### Basics

Let us begin with some examples:

Source

    <define-tag foo>
    This is a simple tag
    </define-tag>
    <define-tag bar endtag=required>
    This is a complex tag
    </define-tag>
    <foo/>
    <bar>Body function</bar>

Output

    This is a simple tag

    This is a complex tag

User defined macros may have attributes like HTML tags. To handle these attributes in replacement text, following conventions have been adopted (mostly derived from Meta-HTML):

- Sequence `%name` is replaced by the command name.
- Attributes are numbered from 0. In replacement text, `%0` is replaced by first argument, `%1` by the 2nd, etc. As there is no limitation on the number of arguments, `%20` is the 21st argument and not the third followed by the 0 letter.

  Source

      <define-tag href>
      <a href="%0">%1</a>
      </define-tag>
      <href http://www.gimp.org "The Gimp" />

  Output

      <a href="http://www.gimp.org">The Gimp</a>

- Sequence `%#` prints number of attributes.
- Sequence `%%` is replaced by `%`, which is useful in nested definitions.

  Source

      <define-tag outer>;;;
      outer, # attributes: %#
      <define-tag inner1>;;;
      inner1, # attributes: %#;;;
      </define-tag>;;;
      <define-tag inner2>;;;
      inner2, # attributes: %%#;;;
      </define-tag>;;;
      <inner1 %attributes and some others />
      <inner2 %attributes and some others />
      </define-tag>
      <outer list attributes />

  Output

      outer, # attributes: 2
      inner1, # attributes: 2
      inner2, # attributes: 5

- Sequence `%attributes` is replaced by the space separated list of attributes.

  Source

      <define-tag mail1>
      <set-var %attributes />
      <get-var name />
      <get-var mail />
      </define-tag>
      <set-var name="" mail="" />
      <mail1 name="Dr. Foo" mail="hello@foo.com" />

  Output

      Dr. Foo
      hello@foo.com

- Sequence `%body` is replaced by the body of a complex macro.

  Source

      <define-tag mail2 endtag=required whitespace=delete>
      <set-var %attributes />
      <a href="mailto:<get-var mail />">%body</a>
      </define-tag>
      <mail2 mail="hello@foo.com">
      <img src="photo.png" alt="Dr. Foo" border=0    />
      </mail2>

  Output

      <a href="mailto:hello@foo.com">
      <img src="photo.png" alt="Dr. Foo" border=0      />
      </a>

- The two forms above accept modifiers. When `%Aattributes` or `%Abody` is used, a newline separated list of attributes is printed.

  Source

      <define-tag show-attributes whitespace=delete>
      <set-var list="%Aattributes" i=0 />
      <foreach attr list>
      <group "%<get-var i />: <get-var attr />" />
      <increment i />
      </foreach>
      </define-tag>
      <show-attributes name="Dr. Foo" mail="hello@foo.com" />

  Output

      %0: name=Dr. Foo%1: mail=hello@foo.com

- Another alternate form is obtained by replacing `A` by `U`, in which case text is replaced but will not be expanded. This does make sense only when macro has been defined with `attributes=verbatim`, otherwise attributes are expanded before replacement.

  Source

      <define-tag show1>
      Before expansion: %Uattributes
      After expansion: %attributes
      </define-tag>
      <define-tag show2 attributes=verbatim>
      Before expansion: %Uattributes
      After expansion: %attributes
      </define-tag>
      <define-tag bar>and here %attributes</define-tag>
      <show1 <bar we go /> />
      <show2 <bar we go /> />

  Output

      Before expansion: and here we go
      After expansion: and here we go

      Before expansion: <bar we go />
      After expansion: and here we go

- Modifiers `A` and `U` can be combined.

Note: Input expansion is completely different in Meta-HTML and in mp4h. With Meta-HTML it is sometimes necessary to use other constructs like `%xbody` and `%qbody`. In order to improve compatibity with Meta-HTML, these constructs are recognized and are interpreted like `%body`. Another feature provided for compatibility reason is the fact that for simple tags `%body` and `%attributes` are equivalent. These features are in the current mp4h version but may disappear in future releases.

### Attributes

Attributes are separated by spaces, tabulations or newlines, and each attribute must be a valid mp4h entity. For instance with the definitions above, `<bar>` can not be an attribute since it must be finished by `</bar>`. But this is valid:

    <foo <foo/> />

or even

    <foo <foo name=src url=ici /> />

In these examples, the `foo` tag has only one argument.

Under certain circumstances it is necessary to group multiple statements into a single one. This can be done with double quotes or with the `group` primitive, e.g.

    <foo "This is the 1st attribute"
               <group and the second /> />

Note: Unlike HTML single quotes can not replace doube quotes for this purpose.

If double quotes appear in an argument, they must be escaped by a backslash `\`.

Source

      <set-var text="Text with double quotes \" inside" />
      <get-var text />

Output

      Text with double quotes " inside

### Macro evaluation

Macros are characterized by

- name
- container status (simple or complex)
- if attributes are expanded or not
- function type (primitive or user defined macro)
- for primitives, address of corresponding code in memory and for user defined macros the replacement text

Characters are read on input until a left angle bracket is found. Then macro name is read. After that attributes are read, verbatim or not depending on how this macro as been defined. And if this macro is complex, its body is read verbatim. When this is finished, some special sequences in replacement text are replaced (like `%body`, `%attributes`, `%0`, `%1`, etc.) and resulting text is put on input stack in order to be rescanned.

Note: By default attributes are evaluated before any replacement.

Consider the following example, to change text in typewriter font:

    <define-tag text-tt endtag=required whitespace=delete>
    <tt>%body</tt>
    </define-tag>

This definition has a major drawback:

Source

    <text-tt>This is an <text-tt>example</text-tt></text-tt>

Output

    <tt>This is an <tt>example</tt></tt>

We would like the inner tags be removed.

First idea is to use an auxiliary variable to know whether we still are inside such an environment:

    <set-var _text:tt=0 />
    <define-tag text-tt endtag=required whitespace=delete>
    <increment _text:tt />
    <ifeq <get-var _text:tt /> 1 "<tt*>" />
    %body
    <ifeq <get-var _text:tt /> 1 "</tt*>" />
    <decrement _text:tt />
    </define-tag>

(the presence of asterisks in HTML tags is explained in next section).

Source

    <text-tt>This is an <text-tt>example</text-tt></text-tt>

Output

    <tt>This is an example</tt>

But if we use simple tags, as in the example below, our definition does not seem to work. It is because attributes are expanded before they are put into replacement text.

Source

    <define-tag opt><text-tt>%attributes</text-tt></define-tag>
    <opt "This is an <opt example />" />

Output

    <tt>This is an <tt>example</tt></tt>

If we want to prevent this problem we have to forbid attributes expansion with

Source

    <define-tag opt attributes=verbatim>;;;
    <text-tt>%attributes</text-tt>;;;
    </define-tag>
    <opt "This is an <opt example />" />

Output

    <tt>This is an example</tt>

### Expansion flags

When you want to embed some server-side scripting language in your pages, you face up some weird problems, like in

    <a href=<%= $url %>>Hello</a>

The question is how do mp4h know that this input has some extra delimiters? The answer is that mp4h should not try to handle some special delimiters, because it cannot handle all of them (there are ASP, ePerl, PHP,... and some of them are customizable). Now, remember that mp4h is a macro-processor, not an XML parser. So we must focus on macros,and format our input file so that it can be parsed without any problem. Previous example may be written

    <a href="<%= $url %>">Hello</a>

because quotes prevent inner right-angle bracket from closing the `a` tag.

Another common problem is when we need to print only a begin or an end tag alone. For instance it is very desirable to define its own headers and footers with

    <define-tag header>
    <html*>
    <head>
    ... put here some information ....
    </head>
    <body* bgcolor="#ffffff" text="#000000">
    </define-tag>
    <define-tag footer>
    </body*>
    </html*>
    </define-tag>

Asterisks mark these tags as pseudo-simple tags, which means that they are complex HTML tags, but used as simple tags within mp4h because tags would not be well nested otherwise.

This asterisk is called ``trailing star'', it appears at the end of the tag name.

Sometimes HTML tags are not parsable, as in this javascript code:

      ...
        document.write('<*img src="foo.gif"');
        if (text) document.write(' alt="'+text+'"');
        document.write('>');
      ...

The ``leading star'' is an asterisk between left-angle bracket and tag name, which prevents this tag from being parsed.

That said we can now understand what the `--expansion` flag is for. It controls how expansion is performed by mp4h. It is followed by an integer, which is a bit sum of the following values

1 | do not parse unknown tags. When set, HTML tags are not parsed. When unset, HTML tags are parsed, i.e. that attributes and/or body is collected.
2 | unknown tags are assumed being simple. When set, HTML tags are simple by default. When unset, HTML tags are complex by default, unless their attribute contain a trailing slash or a trailing star appear just after tag name (see below).
4 | trailing star in tag name do not make this tag simple. When set, trailing star in tag name has no special effect. When unset, it causes an HTML tag to be simple.
8 | an unmatched end tag closes all previous unmatched begin tags. When set, all missing end closing tags are automatically inserted. When unset, an unmatched end tag is discarded and interpreted as normal text, so processing goes on until matching and tag is found.
16 | interpret backslashes as printf. When set, backslashes before non special characters are removed. When unset, they are preserved.
32 | remove trailing slash in tag attributes. When set, remove trailing slash in tag attributes on output. When unset, they are preserved.
64 | do not remove trailing star in tag name. When set, trailing star after tag name are preserved on output. When unset, they are removed.
128 | do not remove leading star in tag name. When set, leading star before tag name are preserved on output. When unset, they are removed.
256 | do not add a space before trailing slash in tag attributes By default, a space is inserted before trailing slash in tag attributes. When set, this space is not prepended.
1024 | suppress warnings about bad nested tags. When set, warnings about bad nested tags are not displayed. When unset, they are printed on standard error.
2048 | suppress warnings about missing trailing slash. When set, warnings about missing trailing slash are not displayed. When unset, they are printed on standard error.

Run

      mp4h -h

to find default value. Current value matches HTML syntax, and it will tend to zero when XHTML syntax becomes more familiar.

## Author

`Denis Barbier`

Mp4h has its own `homepage`.

## Thanks

Sincere thanks to Brian J. Fox for writing Meta-HTML and Rene Seindal for maintaining this wonderful macro parser called GNU m4.