New syntax

Named capture

XRegExp includes comprehensive support for named capture. Capture names can use the characters A–Z, a–z, 0–9, _, and $ only. Following are the details of XRegExp's named capture syntax:

• Capture: (?<name>⋯)
• Backreference in regex: \k<name>
• Backreference in replacement text: ${name}
• Backreference stored at: result.name
• Backreference numbering: Sequential

You can compare these details to named capture in other regex flavors.

Example code

var repeatedWords = XRegExp("\\b (?<word>[a-z]+) \\s+ \\k<word> \\b", "gix");
var input = "The the test data.";

// Check if input contains repeated words
var hasRepeatedWords = repeatedWords.test(input); // -> true

// Use the regex to remove repeated words
var output = input.replace(repeatedWords, "${word}"); // -> "The test data."

var url = "http://yahoo.com/path/to/file?q=1";
var parser = XRegExp("^ (?<scheme> [^:/?]+ ) ://   # aka protocol   \n\
                        (?<host>   [^/?]+  )       # domain name/IP \n\
                        (?<path>   [^?]*   ) \\??  # optional path  \n\
                        (?<query>  .*      )       # optional query   ", "x");

var parts = parser.exec(url);
/* ->
parts: ["http://yahoo.com/path/to/file?q=1", "http", "yahoo.com", "/path/to/file", "q=1"]
parts.scheme: "http"
parts.host: "yahoo.com"
parts.path: "/path/to/file"
parts.query: "q=1"
*/

// Named backreferences available in replacement functions as properties of the first argument
url = url.replace(parser, function (match) {
	return match.replace(match.host, "microsoft.com");
});
// -> "http://microsoft.com/path/to/file?q=1"

Annotations

• Rationale: Named capture can help make regular expressions and related code self-documenting, and thereby easier to read and use.
• Compatibility: The syntax is illegal in ECMAScript 3 and hence should not cause problems. Backreferences with no corresponding capturing group are treated as literal strings or tokens, in congruity with native handling for numbered backreferences.
• Compatibility with deprecated features: XRegExp's named capture functionality does not support the lastMatch property of the global RegExp object or the RegExp.prototype.compile method, since those features were deprecated in JavaScript 1.5.
• Prior art: Comes from Python (feature) and .NET (syntax).

Inline comments

Inline comments use the syntax (?#comment). They are an alternative to the line comments allowed in free-spacing mode.

Example code

var regex = XRegExp("(?#month)\\d\\d?[-/. ](?#day)\\d\\d?[-/. ](?#year)(?:\\d\\d){1,2}");
var isDate = regex.test("04/20/2008"); // -> true

Annotations

• Rationale: Comments make regular expressions more readable.
• Compatibility: No known problems with this syntax; it is illegal in native ECMAScript 3 regular expressions.
• Prior art: The syntax comes from Perl. It is also available in .NET, PCRE, Python, Ruby, and Tcl, among other regular expression flavors.

Leading mode modifier

TODO: Document me.