Mimicking Lookbehind in JavaScript

Unlike lookaheads, JavaScript doesn't support regex lookbehind syntax. That's unfortunate, but I'm not content with just resigning to that fact. Following are three ways I've come up with to mimic lookbehinds in JavaScript.

For those not familar with the concept of lookbehinds, they are zero-width assertions which, like the more specific \b, ^, and $ metacharacters, don't actually consume anything — they just match a position within text. This can be a very powerful concept. Read this first if you need more details.

Mimicking lookbehind with the replace method and optional capturing groups

This first approach is not much like a real lookbehind, but it might be "good enough" in some simple cases. Here are a few examples:

// Mimic leading, positive lookbehind like replace(/(?<=es)t/g, 'x')
var output = 'testt'.replace(/(es)?t/g, function($0, $1){
	return $1 ? $1 + 'x' : $0;
// output: tesxt

// Mimic leading, negative lookbehind like replace(/(?<!es)t/g, 'x')
var output = 'testt'.replace(/(es)?t/g, function($0, $1){
	return $1 ? $0 : 'x';
// output: xestx

// Mimic inner, positive lookbehind like replace(/\w(?<=s)t/g, 'x')
var output = 'testt'.replace(/(?:(s)|\w)t/g, function($0, $1){
	return $1 ? 'x' : $0;
// output: text

Unfortunately, there are many cases where lookbehinds can't be mimicked using this construct. Here's one example:

// Trying to mimic positive lookbehind, but this doesn't work
var output = 'ttttt'.replace(/(t)?t/g, function($0, $1){
	return $1 ? $1 + 'x' : $0;
// output: txtxt
// desired output: txxxx

The problem is that the regexes are relying on actually consuming the characters which should be within zero-width lookbehind assertions, then simply putting back the match unviolated (an effective no-op) if the backreferences contain or don't contain a value. Since the actual matching process here doesn't work anything like real lookbehinds, this only works in a limited number of scenarios. Additionally, it only works with the replace method, since other regex-related methods don't offer a mechanism to dynamically "undo" matches. However, since you can run arbitrary code in the replacement function, it does offer a limited degree of flexibility.

Mimicking lookbehind through reversal

The next approach uses lookaheads to mimic lookbehinds, and relies on manually reversing the data and writing your regex backwards. You'll also need to write the replacement value backwards if using this with the replace method, flip the match index if using this with the search method, etc. If that sounds a bit confusing, it is. I'll show an example in a second, but first we need a way to reverse our test string, since JavaScript doesn't provide this capability natively.

String.prototype.reverse = function () {
	return this.split('').reverse().join('');

Now let's try to pull this off:

// Mimicking lookbehind like (?<=es)t
var output = 'testt'.reverse().replace(/t(?=se)/g, 'x').reverse();
// output: tesxt

That actually works quite nicely, and allows mimicking both positive and negative lookbehind. However, writing a more complex regex with all nodes reversed can get a bit confusing, and since lookahead is used to mimic lookbehind, you can't mix what you intend as real lookaheads in the same pattern.

Note that reversing a string and applying regexes with reversed nodes can actually open up entirely new ways to approach a pattern, and in a few cases might make your code faster, even with the overhead of reversing the data. I'll have to save the efficiency discussion for another day, but before moving on to the third lookbehind-mimicking approach, here's one example of a new pattern approach made possible through reversal.

In my last post, I used the following code to add commas every three digits from the right for all numbers which are not preceded by a dot, letter, or underscore:

String.prototype.commafy = function () {
	return this.replace(/(^|[^\w.])(\d{4,})/g, function($0, $1, $2) {
		return $1 + $2.replace(/\d(?=(?:\d\d\d)+(?!\d))/g, '$&,');

Here's an alternative implementation:

String.prototype.commafy = function() {
	return this.
		replace(/\d\d\d(?=\d)(?!\d*[a-z._])/gi, '$&,').

I'll leave the analysis for your free time.

Finally, we come to the third lookbehind-mimicking approach:

Mimicking lookbehind using a while loop and regexp.lastIndex

This last approach has the following advantages:

  • It's easier to use (no need to reverse your data and regex nodes).
  • It allows lookahead and lookbehind to be used together.
  • It allows you to more easily automate the mimicking process.

However, the trade off is that, in order to avoid interfering with standard regex backtracking, this approach only allows you to use lookbehinds (positive or negative) at the very start and/or end of your regexes. Fortunately, it's quite common to want to use a lookbehind at the start of a regex.

If you're not already familiar with the exec method available for RegExp objects, make sure to read about it at the Mozilla Developer Center before continuing. In particular, look at the examples which use exec within a while loop.

Here's a quick implementation of this approach, in which we'll actually toy with the regex engine's bump-along mechanism to get it to work as we want:

var data = 'ttttt',
	regex = /t/g,
	replacement = 'x',
	lastLastIndex = 0,
	output = '';

regex.x = {
	gRegex: /t/g,
	startLb: {
		regex: /t$/,
		type: true

function lookbehind (data, regex, match) {
	return (
		(regex.x.startLb ? (regex.x.startLb.regex.test(data.substring(0, match.index)) === regex.x.startLb.type) : true) &&
		(regex.x.endLb ? (regex.x.endLb.regex.test(data.substring(0, regex.x.gRegex.lastIndex)) === regex.x.endLb.type) : true)

while (match = regex.x.gRegex.exec(data)) {
	/* If the match is preceded/not by start lookbehind, and the end of the match is preceded/not by end lookbehind */
	if (lookbehind(data, regex, match)) {
		/* replacement can be a function */
		output += data.substring(lastLastIndex, match.index) + match[0].replace(regex, replacement);
			lastLastIndex = regex.gRegex.lastIndex;
	/* If the inner pattern matched, but the leading or trailing lookbehind failed */
	} else {
		output += match[0].charAt(0);
		/* Set the regex to try again one character after the failed position, rather than at the end of the last match */
		regex.x.gRegex.lastIndex = match.index + 1;
	lastLastIndex = regex.x.gRegex.lastIndex;
output += data.substring(lastLastIndex);

// output: txxxx

That's a fair bit of code, but it's quite powerful. It accounts for using both a leading and trailing lookbehind, and allows using a function for the replacement value. Also, this could relatively easily be made into a function which accepts a string for the regex using normal lookbehind syntax (e.g., "(?<=x)x(?<!x)"), then splits it into the various parts in needs before applying it.


  • regex.x.gRegex should be an exact copy of regex, with the difference that it must use the g flag whether or not regex does (in order for the exec method to interact with the while loop as we need it to).
  • regex.x.startLb.type and regex.x.endLb.type use true for "positive," and false for "negative."
  • regex.x.startLb.regex and regex.x.endLb.regex are the patterns you want to use for the lookbehinds, but they must contain a trailing $. The dollar sign in this case does not mean end of the data, but rather end of the data segment they will be tested against.

If you're wondering why there hasn't been any discussion of fixed- vs. variable-length lookbehinds, that's because none of these approaches have any such limitations. They support full, variable-length lookbehind, which no regex engines I know of other than .NET and JGsoft (used by products like RegexBuddy) are capable of.

In conclusion, if you take advantage of all of the above approaches, regex lookbehind syntax can be mimicked in JavaScript in the vast majority of cases. Make sure to take advantage of the comment button if you have feedback about any of this stuff.

Update 2012-04: See my followup blog post, JavaScript Regex Lookbehind Redux, where I've posted a collection of short functions that make it much easier to simulate leading lookbehind.

67 thoughts on “Mimicking Lookbehind in JavaScript”

  1. I mimic lookbehind assertions this way:
    str.replace(/(foo)bar/, ‘$1baz’);
    str.replace(/([^f][^o][^o])bar/, ‘$1baz’);

  2. telega, it’s a bit of a stretch to say those mimic lookbehinds. They’re similar to the “Mimicking lookbehind with replace() and optional capturing groups” examples (which, as I’ve already stated, work very differently from real lookbehinds), but they’re less flexible. They also have the same limitation of only being usable with the replace() method.

    BTW, your latter regex is flawed in that, e.g., it will not match “forbar”. You can fix it with something like /(?!foo)(.{3})bar/

  3. Could you verify my regexp? I have a string containing html. I need to replace all occurrences of ‘<‘ and ‘>’ except for <br>, <br/> and <br some attributes >. I did this (including your fix):

  4. Change to:


    But be aware that this only replaces paired “<” and “>” characters.

  5. Any advice on looking behind to match > where it is inside of HTML content without matching the close of actual html tags. Likewise for ” and &.

    I’ve had a heck of a time trying to come up with something.

    Sample input might be (if this comes out right after encoding)

    <body>This is a &lt;test>.

  6. @Steven, I’m going to guess that you consider > characters to be separate from HTML tags if they are not preceded by a < character with no > characters in between. That would be something like (?<!<[^>]*)>

    If you don’t have real lookbehind available with your regex flavor, use the techniques outlined in this post to adapt it for your needs as appropriate.

    I’m not sure what you mean by ‘Likewise for ” and &’, since those characters don’t close HTML tags like > does.

    In any case, if you have questions which are unrelated to my blog posts, I would recommend you direct them to another location such as regexadvice.com or the RegexBuddy forums (both of which I hang out at from time to time).

  7. Just chiming in from EE (posting as mreuring there) to acknowledge that I particularly like your way outside the box thinking of reversing the string and using look-aheads on it, just to mimick look-behinds!
    Nice post!

  8. Hey,

    I’m trying to implement your negative look-behind solution in one of my current projects, but I am having a little trouble. I have a textarea with a bunch of junk in it. Out of that junk I need to match both a URL and an email address. The only problem is, if the email is before the url, then the host portion of the email is used as the URL. So I would like mimic this type of look-behind: (?>!\@) . Any ideas?

  9. I think you meant (?<!@). Unfortunately, you haven’t given enough info to base things on. According to the mimicking-via-reversal approach, that segment of the regex would become (?!@).

    Rather than mimicking lookbehind, an alternative approach might be including the preceding character in the match and checking what it is outside of the regex. You might also be able to simply merge the two regexes using top-level alternation, with the email address part of the regex coming first.

  10. >> “1283475822347”.replace(/(\d{1,3})(?=(?:\d{3})+(?!\d))/g,”$1,”);

  11. @foo, that’s not equivalent to the solution I posted. If you keep just the equivalent part from my code, you get replace(/\d(?=(?:\d\d\d)+(?!\d))/g,'$&,'), which is very similar to what you posted, except that it’s better (shorter, faster, etc.). 😉

  12. Since I’m a n00b and couldn’t figure out how to use your functions for match, rather than replace, I did the following:

    var source = “Hello Something Goodbye Something Whatever Something”;

    //I want to match ‘Something’, only if it has ‘Goodbye ‘ before it.

    var match = source.match(/Goodbye Something/).join().match(/Something/).join();

    I’ve only been messing with Javascript for a few weeks, so I have no idea how limited the above is in efficiency or usability in more complicated patterns, but it works for what I’m doing 🙂

  13. We can’t do:

    htmlstr = htmlstr.replace(/(((?![“>]))((?:ftp:\/\/|http:\/\/)[^”\r\n })]+|\bwww\.[^”\r\n })]+))/gm, ‘$1‘);

    because Javascript doesn’t do lookbehind. But we can do:

    htmlstr = htmlstr.replace(/(^|[^”>])((?:ftp:\/\/|http:\/\/)[^”\r\n })]+|\bwww\.[^”\r\n })]+)/gm, ‘$1$2‘);

    [not a perfect URL match, but good enough for most]
    Regexes make me happy.

  14. While limitations are often a source of frustration, dreaming up ingenious workarounds to overcome them is one of the joys of writing code. Thanks for reminding us all of this fact!

  15. Good Day,

    Actually, I was searching thru the book and also look at your regex howevever, due to my limited understanding of implementation of regex in JavaScript, found it rather hard to understand.

    I already have a Perl regex of :”(?<=\=)\w+(?=;|,)" to trap the string between the "=" sign and the "," or ";".

    C132 C 1=UNNAMED_1_CON18_I1_A,

    Would it be possible to should how could I store the content between the detected by the Perl Regex into a Variable for further processing

  16. I tried your method but didnt get the results I was looking for…
    When I run this expression: /(?&lt![#\da-zA-z])(\d+)f/g
    On this string: “1.2f; #00ff00”
    I should get: “1.2; #00ff00”

    The ‘f’ shoudl get removed.

    Any ideas?

  17. hello,
    I want a regext javascript to retrieve the numbers followed by a:
    * – Or | and
    * -) (Closing parenthesis)
    * – No (end of string)

    and preceded by:
    * – Or | and
    * – ( (Open parenthesis)
    * – Nothing (beginning of string)
    thank you in advance

  18. Actually,
    I have this regexp but in php:
    (? <= and | or | \( ) $myString (? = (and | or | \\) )?)

  19. Have you figured out how to mimic lookbehinds in splits? For example how would one go about converting the following:


    To work in Javascript?

    Or is this a lost cause?

  20. Hi Steven ..

    Using this concept, what do you think about getting “else’s”, testing if before that, has a “if” expression?

    Like this:

    Regexp: /(if(.|\n)*?)?else/ig

    Delphi Code to Test:

    case m of
    2: if (y mod 4) = 0 then begin
    if d>29 then vd:=false
    else vd:=true ;
    else begin
    if d>28 then vd:=false
    else vd:=true ;

    4,6,8,9,11: if d>30 then vd:=false
    else vd:=true ;
    vd:=true ;

    Important: I wanna get all “else”, and them test if this “else” have a “if” before that.

    But using this regexp i can’t get if have a “if” inside “if”

    Sorry, if that sounds a bit confusing.

    Do you have a better solution?

    This sounds like a challange! 😉

    Tks man!

  21. just use string.replace optional matches and a function to filter:

    var x = /(\bvar\b)?([\;\{\s+\}]*)(foodle)/gm

    var str = “var foodle = 2; foodle(); “;
    var x = str.replace(x,function () {
    if (arguments[1]) return arguments[0]
    return arguments[0].replace(“foodle”,arguments[1]+”asdfg”)

  22. Or if you want to generate the matches rather than do a replace then just store the results in an object etc and return arguments[0]

  23. What would be useful would be to provide the equivalent of \b(\w+)\b for getting all words in document for unicode. Xregex provides \p{L} for matching unicode characters and \P{L} as the negation so it should be possible to do something like the following


    to get all unicode words

  24. I think the first line in the else branch of the while loop in the 3rd method needs to be changed from

    output += match[0].charAt(0)


    output += data.substring(lastLastIndex, match.index) + match[0].charAt(0)

    Otherwise, if the regex.x.gRegex.exec(data) consumes multiple characters via non-matches, those characters won’t be included in the output.

  25. Also in the 3rd method, ‘regex.x.gRegex’ should NOT be an exact copy of ‘regex’. ‘regex.x.gRegex’ should be the intended regex, including any any lookAHEADS. ‘regex’ should be a copy of ‘regex.x.gRegex’ except that any positive lookaheads whose values won’t be in a match of ‘regex.x.gRegex’ (e.g., any trailing lookahead) should be excluded. Otherwise the match[0].replace(regex, replacement) call will fail because ‘regex’ includes a positive lookahead but the characters that lead to the match aren’t included in match[0].

  26. nothing like writing a blog post on doing something regex, to spark an entire comment thread asking to verify their regex like it’s stackoverflow or something

  27. Please, help!
    I need to extract Text Goes Here from (without angle brackets) using Javascript/VBscript regexp. Neither of them support look behind (to strip off the first angular bracket). What would be the alternative?

    Thank you in advance!

  28. I have the following problem scenario:

    Objective: Get rid off all numerics except when the digits are preceded by the term ‘ITEMSTARTMARKER’

    Sample Text: ITEMSTARTMARKER 8 components interest rates 77 term debt ITEMSTARTMARKER 16 fiscal year 2017

    I use the negative lookahead (?<!ITEMSTARTMARKER )\d+ but it doesn't work when more than one digit is followed by ITEMSTARTMARKER. It matches the 6 in ITEMSTARTMARKER 16.

    Any ideas how I can get around it?

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