#46904 [Opn]: preg_match() example #4 is wrong

ID: 46904 Updated by: philip@php.net Reported By: joe at digg dot com Status: Open Bug Type: Documentation problem Operating System: Debian GNU/Linux PHP Version: Irrelevant New Comment: And for good measure, here's a partial PCRE History in PHP (from changelogs): 5.0.0: 4.5 5.0.5: 5.0 5.1.0: 6.2 5.1.3: 6.6 5.2.0: 6.7 5.2.2: 7.0 5.2.4: 7.2 5.2.5: 7.3 5.2.6: 7.6 5.2.7: 7.8 4.4.9: 7.7 Previous Comments: ------------------------------------------------------------------------ [2008-12-23 19:21:44] felipe@php.net Yes, It's expected. But Ok, we should specify a PCRE minimum version, 7.0 (bundled as of PHP 5.2.2) ------------------------------------------------------------------------ [2008-12-23 19:13:06] joe at digg dot com This isn't bogus. At some point this was NOT valid, but now appears to be valid. In 5.2.6 it works fine, but in 5.2.0 it does NOT: jstump@devwww25:~$ php -v && php -q foo.php PHP 5.2.0-8+etch1 (cli) (built: Mar 8 2007 09:15:48) Copyright (c) 1997-2006 The PHP Group Zend Engine v2.2.0, Copyright (c) 1998-2006 Zend Technologies PHP Warning: preg_match(): Compilation failed: unrecognized character after (?< at offset 3 in /home/jstump/foo.php on line 5 Warning: preg_match(): Compilation failed: unrecognized character after (?< at offset 3 in /home/jstump/foo.php on line 5 ------------------------------------------------------------------------ [2008-12-23 19:07:31] felipe@php.net Says the PCRE documentation: "In PCRE, a subpattern can be named in one of three ways: (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python." ------------------------------------------------------------------------ [2008-12-19 10:16:59] rquadling@php.net According to the help for RegexBuddy ... (?P<name>group) came from Python. The PCRE followed Python's lead. PHP offers the same functionality So, initially you look correct. But, again from the RegexBuddy help ... "The regular expression classes of the .NET framework also support named capture. Unfortunately, the Microsoft developers decided to invent their own syntax, rather than follow the one pioneered by Python. Currently, no other regex flavor supports Microsoft's version of named capture. Here is an example with two capturing groups in .NET style: (? <first>group)(?'second'group). As you can see, .NET offers two syntaxes to create a capturing group: one using sharp brackets, and the other using single quotes. The first syntax is preferable in strings, where single quotes may need to be escaped. The second syntax is preferable in ASP code, where the sharp brackets are used for HTML tags. You can use the pointy bracket flavor and the quoted flavors interchangeably. To reference a capturing group inside the regex, use \k<name> or \k'name'. Again, you can use the two syntactic variations interchangeably." This info is also available on http://www.regular- expressions.info/named.html So, it seems PHP actually supports PCRE/Python's and Microsoft's mechanisms. Ideally we should be reflecting the PCRE route but have a note that other mechanisms are supported. Finally on this (from http://perldoc.perl.org/perlre.html - scroll down to "Capture Buffers"). "Additionally, as of Perl 5.10.0 you may use named capture buffers and named backreferences. The notation is (?<name>...) to declare and \k<name> to reference. You may also use apostrophes instead of angle brackets to delimit the name; and you may use the bracketed \g{name} backreference syntax. It's possible to refer to a named capture buffer by absolute and relative number as well. Outside the pattern, a named capture buffer is available via the %+ hash. When different buffers within the same pattern have the same name, $+{name} and \k<name> refer to the leftmost defined group. (Thus it's possible to do things with named capture buffers that would otherwise require (?? {}) code to accomplish.)" So, there is a differentiation between named captures and named backreferences. (?<name>regex>) is a named capture. You cannot use the name of the capture within the regex or the replace (if search/replacing). So, technically and being ever so slightly picky, the documentation is correct. But really it is incomplete. I'll try and put some more examples in differentiating between named captures and named backreferences. ------------------------------------------------------------------------ [2008-12-19 10:05:34] rquadling@php.net I'm not so sure. Using RegexBuddy to explain the different Regexs ... There seems to be no difference between the 2 forms. (?<name>\w+): (?<digit>\d+) Options: case insensitive; ^ and $ match at line breaks Match the regular expression below and capture its match into backreference with name “name” «(?<name>\w+)» Match a single character that is a “word character” (letters, digits, etc.) «\w+» Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+» Match the characters “: ” literally «: » Match the regular expression below and capture its match into backreference with name “digit” «(?<digit>\d+)» Match a single digit 0..9 «\d+» Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+» (?P<name>\w+): (?P<digit>\d+) Options: case insensitive; ^ and $ match at line breaks Match the regular expression below and capture its match into backreference with name “name” «(?P<name>\w+)» Match a single character that is a “word character” (letters, digits, etc.) «\w+» Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+» Match the characters “: ” literally «: » Match the regular expression below and capture its match into backreference with name “digit” «(?P<digit>\d+)» Match a single digit 0..9 «\d+» Between one and unlimited times, as many times as possible, giving back as needed (greedy) «+» ------------------------------------------------------------------------ The remainder of the comments for this report are too long. 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