Multiple String Replacement Sugar

How many times have you needed to run multiple replacement operations on the same string? It's not too bad, but can get a bit tedious if you write code like this a lot.

str = str.
	replace( /&(?!#?\w+;)/g , '&'    ).
	replace( /"([^"]*)"/g   , '“$1”'     ).
	replace( /</g           , '&lt;'     ).
	replace( />/g           , '&gt;'     ).
	replace( /…/g           , '&hellip;' ).
	replace( /“/g           , '&ldquo;'  ).
	replace( /”/g           , '&rdquo;'  ).
	replace( /‘/g           , '&lsquo;'  ).
	replace( /’/g           , '&rsquo;'  ).
	replace( /—/g           , '&mdash;'  ).
	replace( /–/g           , '&ndash;'  );

A common trick to shorten such code is to look up replacement values using an object as a hash table. Here's a simple implementation of this.

var hash = {
	'<' : '&lt;'    ,
	'>' : '&gt;'    ,
	'…' : '&hellip;',
	'“' : '&ldquo;' ,
	'”' : '&rdquo;' ,
	'‘' : '&lsquo;' ,
	'’' : '&rsquo;' ,
	'—' : '&mdash;' ,
	'–' : '&ndash;'

str = str.
	replace( /&(?!#?\w+;)/g , '&amp;' ).
	replace( /"([^"]*)"/g   , '“$1”'  ).
	replace( /[<>…“”‘’—–]/g , function ( $0 ) {
		return hash[ $0 ];

However, this approach has some limitations.

  • Search patterns are repeated in the hash table and the regular expression character class.
  • Both the search and replacement are limited to plain text. That's why the first and second replacements had to remain separate in the above code. The first replacement used a regex search pattern, and the second used a backreference in the replacement text.
  • Replacements don't cascade. This is another reason why the second replacement operation had to remain separate. I want text like "this" to first be replaced with “this”, and eventually end up as &ldquo;this&rdquo;.
  • It doesn't work in Safari 2.x and other old browsers that don't support using functions to generate replacement text.

With a few lines of String.prototype sugar, you can deal with all of these issues.

String.prototype.multiReplace = function ( hash ) {
	var str = this, key;
	for ( key in hash ) {
		str = str.replace( new RegExp( key, 'g' ), hash[ key ] );
	return str;

Now you can use code like this:

str = str.multiReplace({
	'&(?!#?\\w+;)' : '&amp;'   ,
	'"([^"]*)"'    : '“$1”'    ,
	'<'            : '&lt;'    ,
	'>'            : '&gt;'    ,
	'…'            : '&hellip;',
	'“'            : '&ldquo;' ,
	'”'            : '&rdquo;' ,
	'‘'            : '&lsquo;' ,
	'’'            : '&rsquo;' ,
	'—'            : '&mdash;' ,
	'–'            : '&ndash;'

If you care about the order of replacements, you should be aware that the current JavaScript specification does not require a particular enumeration order when looping over object properties with However, recent versions of the big four browsers (IE, Firefox, Safari, Opera) all use insertion order, which allows this to work as described (from top to bottom). ECMAScript 4 proposals indicate that the insertion-order convention will be formally codified in that standard.

If you need to worry about rogue properties that show up when people mess with Object.prototype, you can update the code as follows:

String.prototype.multiReplace = function ( hash ) {
	var str = this, key;
	for ( key in hash ) {
		if ( hash, key ) ) {
			str = str.replace( new RegExp( key, 'g' ), hash[ key ] );
	return str;

Calling the hasOwnProperty method on Object.prototype rather than on the hash object directly allows this method to work even when you're searching for the string "hasOwnProperty".

Lemme know if you think this is useful.

Remove Nested Patterns with One Line of JavaScript

Here's a neat little trick I came up with for removing nested patterns from a string.

var str = "abc<1<2<>3>4>def";

while (str != (str = str.replace(/<[^<>]*>/g, "")));

// str -> "abcdef"

Notice that the regex in this one-liner doesn't try to deal with nested patterns at all. The while loop's condition replaces instances of <…> (where angled brackets are not allowed in the inner pattern) with an empty string. This repeats from the inside out, until the regex no longer matches. At that point, the result of the replacement is the same as the subject string, and the loop ends.

You can use a similar approach to grab nested patterns rather than delete them, as shown below.

[Edit (6/6/2008): The following code does not correctly handle input like "((a)(b))". If you browse tag recursion on this blog you'll find a variety of other approaches for matching nested constructs that actually work correctly.]

var str = "abc(d(e())f)(gh)ijk()",
    re = /\([^()]*\)/,
    output = [],
    match, parts, last;

while (match = re.exec(str)) {
    parts = match[0].split("\uFFFF");
    if (parts.length < 2) {
        last = output.push(match[0]) - 1;
    } else {
        output[last] = parts[0] + output[last] + parts[1];
    str = str.replace(re, "\uFFFF");

// output -> ["(d(e())f)", "(gh)", "()"]

Since once again we're working from the inside out, reassembling each complete match requires us to mark the position at which the previous deepest-level match was removed. I've used the Unicode escape sequence \uFFFF to mark such positions, because that's a permanently-unassigned code point.

Note that using a negated character class like [^()] to match the inner pattern as shown in the examples here only works correctly if you're using single-character delimiters like (…) or <…>. If you want to match/remove nested patterns that use multi-character delimiters, you can use a regex like /<<(?:(?!<<|>>)[\S\s])*>>/. Just change both instances of << to your left delimiter, and >> to your right delimiter.

Test Your XRegExps with JRX

Cüneyt Yılmaz's JRX is a cool JavaScript regex tester inspired by the RX tool of Komodo IDE. Cüneyt recently added my XRegExp library to his tester, so JRX is now a nice and easy way to test XRegExp's singleline and extended modes, as well as named capture and other XRegExp-provided syntax. Check it out!

As for XRegExp, it has recently been upgraded to v0.5.2, which resolved a corner-case bug involving XRegExp.matchRecursive. See the changelog for details.

I'll take this opportunity to highlight some of my other favorite online regex testers. I've actively looked for these kinds of apps over the years and have seen more than 50 of them. Odds are you'll find something new here.

Edit (2008-06-18): Updated the list with a couple that have come out very recently.

  • RegexPal — My own JavaScript regex tester. It includes real-time regex syntax and match highlighting. Although RegexPal uses XRegExp to provide the singleline option, unlike JRX it uses JavaScript regex syntax without the XRegExp syntax extensions.
  • regex — Simple name, simple interface. Great set of flavor support (JavaScript, Perl, Python, PCRE, POSIX ERE).
  • Regexp Editor — Java regex tester with regex syntax highlighting.
  • RegExr — ActionScript regex tester with regex syntax highlighting.
  • reWork — JavaScript regex workbench.
  • reAnimator — Fun app for visualizing regex FSAs.
  • RegexMate — JavaScript regex console.
  • The REWizard — IE only, but offers regex building tools and an interesting visualization.
  • MyRegexTester — Includes code generation and plain-text explanations (via the YAPE::Regex::Explain Perl module).
  • Regular Expression Analyzer — Real-time regex explanation tree that mostly emulates Java, JavaScript, and Perl flavors. Its regex parsing code is very readable.
  • Nregex — .NET regex tester.
  • Rubular — Ruby regex tester.

Have fun!

XRegExp 0.5 Released!

Update: This version of XRegExp is outdated. See for the latest, greatest version.

If you haven't seen the prior versions, XRegExp is an MIT-licensed JavaScript library that provides an augmented, cross-browser implementation of regular expressions, including support for additional modifiers and syntax. Several convenience methods and a new, powerful recursive-construct parser that uses regex delimiters are also included.

Here's what you get beyond the standard JavaScript regex features:

  • Added regex syntax:
    • Comprehensive named capture support. (Improved)
    • Comment patterns: (?#…). (New)
  • Added regex modifiers (flags):
    • s (singleline), to make dot match all characters including newlines.
    • x (extended), for free-spacing and comments.
  • Added awesome:
    • Reduced cross-browser inconsistencies. (More)
    • Recursive-construct parser with regex delimiters. (New)
    • An easy way to cache and reuse regex objects. (New)
    • The ability to safely embed literal text in your regex patterns. (New)
    • A method to add modifiers to existing regex objects.
    • Regex call and apply methods, which make generically working with functions and regexes easier. (New)

All of this can be yours for the low, low price of 2.4 KB. smile Version 0.5 also introduces extensive documentation and code examples.

If you're using a previous version, note that there are a few non-backward compatible changes for the sake of strict ECMA-262 Edition 3 compliance and compatibility with upcoming ECMAScript 4 changes.

  • The XRegExp.overrideNative function has been removed, since it is no longer possible to override native constructors in Firefox 3 or ECMAScript 4 (as proposed).
  • Named capture syntax has been changed from (<name>…) to (?<name>…), which is the standard in most regex libraries and under consideration for ES4. Named capture is now always available, and does not require the k modifier.
  • Due to cross-browser compatibility issues, previous versions enforced that a leading, unescaped ] within a character class was treated as a literal character, which is how things work in most regex flavors. XRegExp now follows ECMA-262 Edition 3 on this point. [] is an empty set and never matches (this is enforced in all browsers).

Get it while it's hot! Check out the new XRegExp documentation and source code.

An IE lastIndex Bug with Zero-Length Regex Matches

The bottom line of this blog post is that Internet Explorer incorrectly increments a regex object's lastIndex property after a successful, zero-length match. However, for anyone who isn't sure what I'm talking about or is interested in how to work around the problem, I'll describe the issue with examples of iterating over each match in a string using the RegExp.prototype.exec method. That's where I've most frequently encountered the bug, and I think it will help explain why the issue exists in the first place.

First of all, if you're not already familiar with how to use exec to iterate over a string, you're missing out on some very powerful functionality. Here's the basic construct:

var	regex = /.../g,
	subject = "test",
	match = regex.exec(subject);

while (match != null) {
	// matched text: match[0]
	// match start: match.index
	// match end: regex.lastIndex
	// capturing group n: match[n]


	match = regex.exec(subject);

When the exec method is called for a regex that uses the /g (global) modifier, it searches from the point in the subject string specified by the regex's lastIndex property (which is initially zero, so it searches from the beginning of the string). If the exec method finds a match, it updates the regex's lastIndex property to the character index at the end of the match, and returns an array containing the matched text and any captured subexpressions. If there is no match from the point in the string where the search started, lastIndex is reset to zero, and null is returned.

You can tighten up the above code by moving the exec method call into the while loop's condition, like so:

var	regex = /.../g,
	subject = "test",

while (match = regex.exec(subject)) {

This cleaner version works essentially the same as before. As soon as exec can't find any further matches and therefore returns null, the loop ends. However, there are a couple cross-browser issues to be aware of with either version of this code. One is that if the regex contains capturing groups which do not participate in the match, some values in the returned array could be either undefined or an empty string. I've previously discussed that issue in depth in a post about what I called non-participating capturing groups.

Another issue (the topic of this post) occurs when your regex matches an empty string. There are many reasons why you might allow a regex to do that, but if you can't think of any, consider cases where you're accepting regexes from an outside source. Here's a simple example of such a regex:

var	regex = /^/gm,
	subject = "A\nB\nC",
	endPositions = [];

while (match = regex.exec(subject)) {

You might expect the endPositions array to be set to [0,2,4], since those are the character positions for the beginning of the string and just after each newline character. Thanks to the /m modifier, those are the positions where the regex will match; and since the regex matches empty strings, regex.lastIndex should be the same as match.index. However, Internet Explorer (tested with v5.5–7) sets endPositions to [1,3,5]. Other browsers will go into an infinite loop until you short-circuit the code.

So what's going on here? Remember that every time exec runs, it attempts to match within the subject string starting at the position specified by the lastIndex property of the regex. Since our regex matches a zero-length string, lastIndex remains exactly where we started the search. Therefore, every time through the loop our regex will match at the same position—the start of the string. Internet Explorer tries to be helpful and avoid this situation by automatically incrementing lastIndex when a zero-length string is matched. That might seem like a good idea (in fact, I've seen people adamantly argue that is a bug that Firefox does not do the same), but it means that in Internet Explorer the lastIndex property cannot be relied on to accurately determine the ending position of a match.

We can correct this situation cross-browser with the following code:

var	regex = /^/gm,
	subject = "A\nB\nC",
	endPositions = [];

while (match = regex.exec(subject)) {
	var zeroLengthMatch = !match[0].length;
	// Fix IE's incorrect lastIndex
	if (zeroLengthMatch && regex.lastIndex > match.index)


	// Avoid an infinite loop with zero-length matches
	if (zeroLengthMatch)

You can see an example of the above code in the cross-browser split method I posted a while back. Keep in mind that none of the extra code here is needed if your regex cannot possibly match an empty string.

Another way to deal with this issue is to use String.prototype.replace to iterate over the subject string. The replace method moves forward automatically after zero-length matches, avoiding this issue altogether. Unfortunately, in the three biggest browsers (IE, Firefox, Safari), replace doesn't seem to deal with the lastIndex property except to reset it to zero. Opera gets it right (according to my reading of the spec) and updates lastIndex along the way. Given the current situation, you can't rely on lastIndex in your code when iterating over a string using replace, but you can still easily derive the value for the end of each match. Here's an example:

var	regex = /^/gm,
	subject = "A\nB\nC",
	endPositions = [];

subject.replace(regex, function (match) {
	// Not using a named argument for the index since capturing
	// groups can change its position in the list of arguments
	var	index = arguments[arguments.length - 2],
		lastIndex = index + match.length;


That's perhaps less lucid than before (since we're not actually replacing anything), but there you have it… two cross-browser ways to get around a little-known issue that could otherwise cause tricky, latent bugs in your code.