# FBB::Pattern(3bobcat)

## Pattern matcher
(libbobcat-dev_6.07.01)

## 2005-2025

## NAME

FBB::Pattern - Performs RE pattern matching

## SYNOPSIS

#include <bobcat/pattern>
Linking option: -lbobcat

## DESCRIPTION

Pattern objects may be used for Regular Expression (RE) pattern matching. The class is a wrapper around the regcomp(3) family of functions. By default it uses `extended regular expressions', requiring you to escape multipliers and bounding-characters when they should be interpreted as ordinary characters (i.e., *, +, ?, ^, $, |, (, ), [, ], {, } should be escaped when used as literal characters).

The Pattern class supports the use of the following (Perl-like) special escape sequences:
\b - indicating a word-boundary
\d - indicating a digit ([[:digit:]]) character
\s - indicating a white-space ([:space:]) character
\w - indicating a word ([:alnum:]) character

The corresponding capitals (e.g., \W) define the complementary character sets. The capitalized character set shorthands are not expanded inside explicit character-classes (i.e., [ ... ] constructions). So [\W] represents a set of two characters: \ and W.

As the backslash (\) is treated as a special character it should be handled carefully. Pattern converts the escape sequences \d \s \w (and outside of explicit character classes the sequences \D \S \W) to their respective character classes. All other escape sequences are kept as-is, and the resulting regular expression is offered to the pattern matching compilation function regcomp(3). This function interprets escape sequences. Consequently some care should be exercised when defining patterns containing escape sequences. Here are the rules:

- Special escape sequences (like \d) are converted to character classes. E.g.,

          ---------------------------------------------------------
          Specify:    Converts to:    regcomp uses:      Matches:
          ---------------------------------------------------------
          \d          [[:digit:]]     [[:digit:]]        3
          ---------------------------------------------------------

- Ordinary escape sequences (like \x) are kept as-is. E.g.,

          ---------------------------------------------------------
          Specify:    Converts to:    regcomp uses:      Matches:
          ---------------------------------------------------------
          \x          \x              x                  x
          ---------------------------------------------------------

- To specify literal escape sequences, Raw String Literals are advised, as they don't require doubling escape sequences. E.g., the following regular expression matches an (alpha-numeric) word, followed by optional blanks, a colon, more optional blanks and a (decimal) number:

          R"((\w+)\s*:\s*\d+)"

## NAMESPACE

FBB
All constructors, members, operators and manipulators, mentioned in this man-page, are defined in the namespace FBB.

## INHERITS FROM

-

## TYPEDEF

- Pattern::Position:
  A nested type representing the offsets of the first character and the offset beyond the last character of the matched text or indexed subexpression, defined as std::pair<std::string::size_type, std::string::size_type>.

## CONSTRUCTORS

- Pattern():
  The default constructor defines no pattern, but is available as a placeholder for, e.g., containers requiring default constructors. A Pattern object thus constructed cannot be used to match patterns, but can be the lvalue in assignments where another Pattern object is the rvalue. However, it can receive a pattern using the member setPattern() (see below). An FBB::Exception object is thrown if the object could not be constructed.
- Pattern(std::string const &pattern, bool caseSensitive = true, size_t nSub = 10, int options = REG_EXTENDED | REG_NEWLINE):
  This constructor compiles pattern, preparing the Pattern object for pattern matches. The second parameter determines whether case sensitive matching will be used (the default) or not. Subexpressions are defined by parentheses pairs. Each matching pair defines a subexpression, where the order-number of their opening parentheses determines the subexpression's index. By default at most 10 subexpressions are recognized. The options flags may be:

  REG_EXTENDED:
  Use POSIX Extended Regular Expression syntax when interpreting regex. If not set, POSIX Basic Regular Expression syntax is used.

  REG_NOSUB:
  Support for substring addressing of matches is not required. The nmatch and pmatch parameters to regexec are ignored if the pattern buffer supplied was compiled with this flag set.

  REG_NEWLINE:
  Match-any-character operators don't match a newline.

  A non-matching list ([^...]) not containing a newline does not match a newline.

  Match-beginning-of-line operator (^) matches the empty string immediately after a newline, regardless of whether eflags, the execution flags of regexec, contains REG_NOTBOL.

  Match-end-of-line operator ($) matches the empty string immediately before a newline, regardless of whether eflags contains REG_NOTEOL.

Copy and move constructors (and assignment operators) are available.

## MEMBER FUNCTIONS

All members of std::ostringstream and std::exception are available, as Pattern inherits from these classes.

- std::string before() const:
  Following a successful match, before() returns the text before the matched text.
- std::string beyond() const:
  Following a successful match, beyond() returns the text beyond the matched text.
- size_t end() const:
  Returns the number of matched elements (text and subexpressions). end() is the lowest index value for which position() returns two std::string::npos values (see the position() member function, below).
- void match(std::string const &text, int options = 0):
  Match a string with a pattern. If the text could not be matched, an Exception exception is thrown , using Pattern::match() as its prefix-text.

  Options may be:

  REG_NOTBOL:
  The match-beginning-of-line operator always fails to match (but see the compilation flag REG_NEWLINE above) This flag may be used when different portions of a string are passed to regexec and the beginning of the string should not be interpreted as the beginning of the line.

  REG_NOTEOL:
  The match-end-of-line operator always fails to match (but see the compilation flag REG_NEWLINE)

- std::string matched() const:
  Following a successful match, this function returns the matched text.
- std::string const &pattern() const:
  This member function returns the pattern that is offered to regcomp(3). It returns the content of a static string that is overwritten at each construction of a Pattern object and at each call of the setPattern() member function.
- Pattern::Position position(size_t index) const:
  With index == 0 the fully matched text is returned (identical to matched()). Other index values return the corresponding subexpressions. std::string::npos, std::string::npos is returned if index is at least end() (which may happen at index value 0).
- void setPattern(std::string const &pattern, bool caseSensitive = true, size_t nSub = 10, int options = REG_EXTENDED | REG_NEWLINE):
  This member function installs a new compiled pattern in its Pattern object. This member's parameters are identical to the second constructor's parameters. Refer to that constructor for details about the parameters. Like the constructor, an FBB::Exception exception is thrown if the new pattern could not be compiled.
- void swap(Pattern &other):
  The content of the current object and the other object are swapped.

## OVERLOADED OPERATORS

- std::string operator[](size_t index) const:
  Returns the matched text (for index 0) or the text of a subexpression. An empty string is returned for index values which are at least end().
- Pattern &operator<<(int matchOptions):
  Defines match-options to be used with the following overloaded operator.
- bool operator<<(std::string const &text):
  Performs a match(text, matchOptions) call, catching any exception that might be thrown. If no matchOptions were set using the above overloaded operator, none are used. The options set this way are not `sticky': when necessary, they have to be re-inserted before each new pattern matching. The function returns true if the matching was successful, false otherwise.

## EXAMPLE

    #include "driver.h"

    #include <bobcat/pattern>

    using namespace std;
    using namespace FBB;

    #include <algorithm>
    #include <cstring>

    void showSubstr(string const &str)
    {
        static int count = 0;

        cout << "String " << ++count << " is '" << str << "'\n";
    }

    void match(Pattern const &patt, string const &text)
    try
    {
         Pattern pattern{ patt };

        pattern.match(text);

        Pattern p3(pattern);

        cout << "before:  " << p3.before() << "\n"
                "matched: " << p3.matched() << "\n"
                "beyond:  " << pattern.beyond() << "\n"
                "end() = " << pattern.end() << '\n';

        for (size_t idx = 0; idx != pattern.end(); ++idx)
        {
            string str = pattern[idx];

            if (str.empty())
                cout << "part " << idx << " not present\n";
            else
            {
                Pattern::Position pos = pattern.position(idx);

                cout << "part " << idx << ": '" << str << "' (" <<
                            pos.first << "-" << pos.second << ")\n";
            }
        }
    }
    catch (exception const &exc)
    {
        cout << exc.what() << '\n';
    }

    int main(int argc, char **argv)
    {
        string patStr = R"(\d+)";

        do
        {
            cout << "Pattern: '" << patStr << "'\n";
            try
            {
                    // by default: case sensitive
                    // use any args. for case insensitive
                Pattern patt(patStr, argc == 1);

                cout << "Compiled pattern: " << patt.pattern() << '\n';

                while (true)
                {
                    cout << "string to match : ";

                    string text;
                    getline(cin, text);
                    if (text.empty())
                        break;
                    cout << "String: '" << text << "'\n";
                    match(patt, text);
                }
            }
            catch (exception const &exc)
            {
                cout << exc.what() << ": compilation failed\n";
            }

            cout << "New pattern: ";
        }
        while (getline(cin, patStr) and not patStr.empty());
    }

## FILES

bobcat/pattern - defines the class interface

## SEE ALSO

bobcat(7), regcomp(3), regex(3), regex(7)

## BUGS

Using Pattern objects as static data members of classes (or as global objects) is potentially dangerous. If the object files defining these static data members are stored in a dynamic library they may not be initialized properly or timely, and their eventual destruction may result in a segmentation fault. This is a well-known problem with static data, see, e.g., http://www.parashift.com/c++-faq-lite/ctors.html#faq-10.15. In situations like this prefer the use of a (shared, unique) pointer to a Pattern, initializing the pointer when, e.g., first used.

## BOBCAT PROJECT FILES

- https://fbb-git.gitlab.io/bobcat/: gitlab project page;

Debian Bobcat project files:

- libbobcat6: debian package containing the shared library, changelog and copyright note;
- libbobcat-dev: debian package containing the static library, headers, manual pages, and developer info;

## BOBCAT

Bobcat is an acronym of `Brokken's Own Base Classes And Templates'.

## COPYRIGHT

This is free software, distributed under the terms of the GNU General Public License (GPL).

## AUTHOR

Frank B. Brokken (f.b.brokken@rug.nl).