[/==============================================================================
    Copyright (C) 2001-2011 Joel de Guzman
    Copyright (C) 2001-2011 Hartmut Kaiser

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
===============================================================================/]

[section:lexer Supported Regular Expressions]

[table Regular expressions support
    [[Expression]   [Meaning]]
    [[`x`]          [Match any character `x`]]
    [[`.`]          [Match any except newline (or optionally *any* character)]]
    [[`"..."`]      [All characters taken as literals between double quotes, except escape sequences]]
    [[`[xyz]`]      [A character class; in this case matches `x`, `y` or `z`]]
    [[`[abj-oZ]`]   [A character class with a range in it; matches `a`, `b` any
                     letter from `j` through `o` or a `Z`]]
    [[`[^A-Z]`]     [A negated character class i.e. any character but those in 
                     the class. In this case, any character except an uppercase 
                     letter]]
    [[`r*`]         [Zero or more r's (greedy), where r is any regular expression]]
    [[`r*?`]        [Zero or more r's (abstemious), where r is any regular expression]]
    [[`r+`]         [One or more r's (greedy)]]
    [[`r+?`]        [One or more r's (abstemious)]]
    [[`r?`]         [Zero or one r's (greedy), i.e. optional]]
    [[`r??`]        [Zero or one r's (abstemious), i.e. optional]]
    [[`r{2,5}`]     [Anywhere between two and five r's (greedy)]]
    [[`r{2,5}?`]    [Anywhere between two and five r's (abstemious)]]
    [[`r{2,}`]      [Two or more r's (greedy)]]
    [[`r{2,}?`]     [Two or more r's (abstemious)]]
    [[`r{4}`]       [Exactly four r's]]
    [[`{NAME}`]     [The macro `NAME` (see below)]]
    [[`"[xyz]\"foo"`]  [The literal string `[xyz]\"foo`]]
    [[`\X`]         [If X is `a`, `b`, `e`, `n`, `r`, `f`, `t`, `v` then the 
                     ANSI-C interpretation of `\x`. Otherwise a literal `X` 
                     (used to escape operators such as `*`)]]
    [[`\0`]         [A NUL character (ASCII code 0)]]
    [[`\123`]       [The character with octal value 123]]
    [[`\x2a`]       [The character with hexadecimal value 2a]]
    [[`\cX`]        [A named control character `X`.]]
    [[`\a`]         [A shortcut for Alert (bell).]]
    [[`\b`]         [A shortcut for Backspace]]
    [[`\e`]         [A shortcut for ESC (escape character `0x1b`)]]
    [[`\n`]         [A shortcut for newline]]
    [[`\r`]         [A shortcut for carriage return]]
    [[`\f`]         [A shortcut for form feed `0x0c`]]
    [[`\t`]         [A shortcut for horizontal tab `0x09`]]
    [[`\v`]         [A shortcut for vertical tab `0x0b`]]
    [[`\d`]         [A shortcut for `[0-9]`]]
    [[`\D`]         [A shortcut for `[^0-9]`]]
    [[`\s`]         [A shortcut for `[\x20\t\n\r\f\v]`]]
    [[`\S`]         [A shortcut for `[^\x20\t\n\r\f\v]`]]
    [[`\w`]         [A shortcut for `[a-zA-Z0-9_]`]]
    [[`\W`]         [A shortcut for `[^a-zA-Z0-9_]`]]
    [[`(r)`]        [Match an `r`; parenthesis are used to override precedence 
                     (see below)]]
    [[`(?r-s:pattern)`] [apply option 'r' and omit option 's' while interpreting pattern.
Options may be zero or more of the characters 'i' or 's'.
'i' means case-insensitive. '-i' means case-sensitive.
's' alters the meaning of the '.' syntax to match any single character whatsoever.
'-s' alters the meaning of '.' to match any character except '`\n`'.]]
    [[`rs`]         [The regular expression `r` followed by the regular 
                     expression `s` (a sequence)]]
    [[`r|s`]        [Either an `r` or and `s`]]
    [[`^r`]         [An `r` but only at the beginning of a line (i.e. when just 
                     starting to scan, or right after a newline has been 
                     scanned)]]
    [[`r`$]         [An `r` but only at the end of a line (i.e. just before a 
                     newline)]]
]

[note POSIX character classes are not currently supported, due to performance issues
when creating them in wide character mode.]

[tip  If you want to build tokens for syntaxes that recognize items like quotes 
      (`"'"`, `'"'`) and backslash (`\`), here is example syntax to get you started. 
      The lesson here really is to remember that both c++, as well as regular 
      expressions require escaping with `\` for some constructs, which can 
      cascade.
      ``
          quote1         = "'";            // match single "'"
          quote2         = "\\\"";         // match single '"'
          literal_quote1 = "\\'";          // match backslash followed by single "'"
          literal_quote2 = "\\\\\\\"";     // match backslash followed by single '"'
          literal_backslash = "\\\\\\\\";  // match two backslashes
      ``
]

[heading Regular Expression Precedence]

* `rs` has highest precedence
* `r*` has next highest (`+`, `?`, `{n,m}` have the same precedence as `*`)
* `r|s` has the lowest precedence

[heading Macros]

Regular expressions can be given a name and referred to in rules using the
syntax `{NAME}` where `NAME` is the name you have given to the macro.  A macro
name can be at most 30 characters long and must start with a `_` or a letter.
Subsequent characters can be `_`, `-`, a letter or a decimal digit.

[endsect]