XRegExp 2.0 is coming soon. Get the Release Candidate at GitHub. See the list of changes in the Roadmap.

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

A mode modifier uses the syntax (?imsx)—where imsx is any combination of one or more of these allowed flags—and provides an alternative way to enable the specified flags. XRegExp allows the use of a mode modifier only at the very beginning of a pattern.

Example code

var regex = XRegExp("(?im)^[a-z]+$");
regex.ignoreCase; // -> true
regex.multiline; // -> true

function test (str, pattern) {
	return XRegExp(pattern).test(str);
}
test("Line1\nLine2", "(?sx)^ .+ $"); // -> true

Flags g (global) and y (sticky) cannot be set via a mode modifier.

Annotations

• Rationale: Mode modifiers allow you to enable flags in situations where a regex pattern can be provided as a string only.
• Compatibility: No known problems with this syntax; it is illegal in native ECMAScript 3 regular expressions.
• Compatibility with other regex flavors: Some regex flavors support the use of multiple mode modifiers anywhere in a pattern, and allow extended syntax for unsetting flags via (?-i), simultaneously setting and unsetting flags via (?i-m), and enabling flags for subpatterns only via (?i:…). XRegExp does not support these extended options.
• Prior art: The syntax comes from Perl. It is also available in .NET, Java, PCRE, Python, Ruby, and Tcl, among other regular expression flavors.