XRegExp 0.2: Now With Named Capture

Update: A beta version of XRegExp 0.3 is now available as part of the RegexPal download package.

JavaScript's regular expression flavor doesn't support named capture. Well, says who? XRegExp 0.2 brings named capture support, along with several other new features. But first of all, if you haven't seen the previous version, make sure to check out my post on , because not all of the documentation is repeated below.

Highlights Comprehensive named capture support (New) Supports regex literals through the addFlags method (New) Free-spacing and comments mode (x) Dot matches all mode (s) Several other minor improvements over v0.1 Named capture

There are several different syntaxes in the wild for named capture. I've compiled the following table based on my understanding of the regex support of the libraries in question. XRegExp's syntax is included at the top.

Library Capture Backreference In replacement Stored at
XRegExp   (<name>…)   \k<name>   ${name}   result.name  
.NET   (?<name>…) (?'name'…)   \k<name> \k'name'   ${name}   Matcher.Groups('name')  
Perl 5.10 (beta)   (?<name>…) (?'name'…)   \k<name> \k'name' \g{name}   $+{name}   ??  
Python   (?P<name>…)   (?P=name)   \g<name>   result.group('name')  
PHP preg (PCRE)   (.NET, Perl, and Python styles)   $regs['name']   $result['name']  

No other major regex library currently supports named capture, although the JGsoft engine (used by products like RegexBuddy) supports both .NET and Python syntax. XRegExp does not use a question mark at the beginning of a named capturing group because that would prevent it from being used in regex literals (JavaScript would immediately throw an "invalid quantifier" error).

XRegExp supports named capture on an on-request basis. You can add named capture support to any regex though the use of the new "k" flag. This is done for compatibility reasons and to ensure that regex compilation time remains as fast as possible in all situations.

Following are several examples of using named capture:

// Add named capture support using the XRegExp constructor var repeatedWords = new XRegExp("\\b (<word> \\w+ ) \\s+ \\k<word> \\b", "gixk"); // Add named capture support using RegExp, after overriding the native constructor XRegExp.overrideNative(); var repeatedWords = new RegExp("\\b (<word> \\w+ ) \\s+ \\k<word> \\b", "gixk"); // Add named capture support to a regex literal var repeatedWords = /\b (<word> \w+ ) \s+ \k<word> \b/.addFlags("gixk"); var data = "The the test data."; // Check if data contains repeated words var hasDuplicates = repeatedWords.test(data); // hasDuplicates: true // Use the regex to remove repeated words var output = data.replace(repeatedWords, "${word}"); // output: "The test data."

In the above code, I've also used the x flag provided by XRegExp, to improve readability. Note that the addFlags method can be called multiple times on the same regex (e.g., /pattern/g.addFlags("k").addFlags("s")), but I'd recommend adding all flags in one shot, for efficiency.

Here are a few more examples of using named capture, with an overly simplistic URL-matching regex (for comprehensive URL parsing, see parseUri):

var url = "http://microsoft.com/path/to/file?q=1"; var urlParser = new XRegExp("^(<protocol>[^:/?]+)://(<host>[^/?]*)(<path>[^?]*)\\?(<query>.*)", "k"); var parts = urlParser.exec(url); /* The result: parts.protocol: "http" parts.host: "microsoft.com" parts.path: "/path/to/file" parts.query: "q=1" */ // Named backreferences are also available in replace() callback functions as properties of the first argument var newUrl = url.replace(urlParser, function(match){ return match.replace(match.host, "yahoo.com"); }); // newUrl: "http://yahoo.com/path/to/file?q=1"

Note that XRegExp's named capture functionality does not support including the lastMatch property of the global RegExp object and the RegExp.prototype.compile() method.

Singleline (s) and extended (x) modes