Simulating lookbehind in JavaScript (take 2)

xregexp-lookbehind2.js

// Simulating infinite-length leading lookbehind in JavaScript. Uses XRegExp.
// Captures within lookbehind are not included in match results. Lazy
// repetition in lookbehind may lead to unexpected results.

(function (XRegExp) {

    function prepareLb(lb) {
        // Allow mode modifier before lookbehind
        var parts = /^((?:\(\?[\w$]+\))?)\(\?<([=!])([\s\S]*)\)$/.exec(lb);
        return {
            // $(?!\s) allows use of (?m) in lookbehind
            lb: XRegExp(parts ? parts[1] + "(?:" + parts[3] + ")$(?!\\s)" : lb),
            // Positive or negative lookbehind. Use positive if no lookbehind group
            type: parts ? parts[2] === "=" : !parts
        };
    }

    XRegExp.execLb = function (str, lb, regex) {
        var pos = 0, match, leftContext;
        lb = prepareLb(lb);
        while (match = XRegExp.exec(str, regex, pos)) {
            leftContext = str.slice(0, match.index);
            if (lb.type === lb.lb.test(leftContext)) {
                return match;
            }
            pos = match.index + 1;
        }
        return null;
    };

    XRegExp.testLb = function (str, lb, regex) {
        return !!XRegExp.execLb(str, lb, regex);
    };

    XRegExp.searchLb = function (str, lb, regex) {
        var match = XRegExp.execLb(str, lb, regex);
        return match ? match.index : -1;
    };

    XRegExp.matchAllLb = function (str, lb, regex) {
        var matches = [], pos = 0, match, leftContext;
        lb = prepareLb(lb);
        while (match = XRegExp.exec(str, regex, pos)) {
            leftContext = str.slice(0, match.index);
            if (lb.type === lb.lb.test(leftContext)) {
                matches.push(match[0]);
                pos = match.index + (match[0].length || 1);
            } else {
                pos = match.index + 1;
            }
        }
        return matches;
    };

    XRegExp.replaceLb = function (str, lb, regex, replacement) {
        var output = "", pos = 0, lastEnd = 0, match, leftContext;
        lb = prepareLb(lb);
        while (match = XRegExp.exec(str, regex, pos)) {
            leftContext = str.slice(0, match.index);
            if (lb.type === lb.lb.test(leftContext)) {
                // Doesn't work correctly if lookahead in regex looks outside of the match
                output += str.slice(lastEnd, match.index) + XRegExp.replace(match[0], regex, replacement);
                lastEnd = match.index + match[0].length;
                if (!regex.global) {
                    break;
                }
                pos = match.index + (match[0].length || 1);
            } else {
                pos = match.index + 1;
            }
        }
        return output + str.slice(lastEnd);
    };

}(XRegExp));


// Test it...

console.log(XRegExp.execLb("Fluffy cat", "(?i)(?<=fluffy\\W+)", XRegExp("(?i)(?<first>c)at")));
// -> ["cat", "c"]
// Result has named backref: result.first -> "c"

console.log(XRegExp.execLb("Fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i));
// -> null

console.log(XRegExp.testLb("Fluffy cat", "(?i)(?<=fluffy\\W+)", /cat/i));
// -> true

console.log(XRegExp.testLb("Fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i));
// -> false

console.log(XRegExp.searchLb("Catwoman's fluffy cat", "(?i)(?<=fluffy\\W+)", /cat/i));
// -> 18

console.log(XRegExp.searchLb("Catwoman's fluffy cat", "(?i)(?<!fluffy\\W+)", /cat/i));
// -> 0

console.log(XRegExp.matchAllLb("Catwoman's cats are fluffy cats", "(?i)(?<=fluffy\\W+)", /cat\w*/i));
// -> ["cats"]

console.log(XRegExp.matchAllLb("Catwoman's cats are fluffy cats", "(?i)(?<!fluffy\\W+)", /cat\w*/i));
// -> ["Catwoman", "cats"]

console.log(XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<=fluffy\\W+)", /cat/ig, "dog"));
// -> "Catwoman's fluffy dog is a cat"

console.log(XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<!fluffy\\W+)", /cat/ig, "dog"));
// -> "dogwoman's fluffy cat is a dog"

console.log(XRegExp.replaceLb("Catwoman's fluffy cat is a cat", "(?i)(?<!fluffy\\W+)", /cat/ig, function ($0) {
    var first = $0.charAt(0);
    return first === first.toUpperCase() ? "Dog" : "dog";
}));
// -> "Dogwoman's fluffy cat is a dog"

This is an alternate version of my previous lookbehind simulation that accepted leading lookbehind and the main regex as one combined pattern. This version does not require the XRegExp.matchRecursive addon.

This code is released under the MIT License.

I've used this code in my blog post JavaScript Regex Lookbehind Redux.

Nice. Until now I didn't know JS regex didn't support lookbehind. Do you know if it supports lookahead assertions? I was testing some regex on the console just last week and couldn't explain why I got a (false) positive match in JS, but not in other languages. I ended up changing strategies and ditching the assertion.

Until now, I'd pretty much assumed that JS had a full regex implementation.

Yes, JavaScript-the-standard supports lookahead, as do all browsers going back to IE 5.5.

I want to match the entire tag that contains some haml databinding script in an attribute
e.g.
'

'

Can this be achieved using matchLb? I don't understand how to match the string starting with < but ending with > and not !>.

sorry. to continue :

e.g.

<table id="r<!=id!>">

I want to match the to the < not preceded by a !.

Is this possible?

Ha! I should have 'known' it would be you to have an alternative for JS ;-)

Are you able to please provide a method for splitting that supports lookbehind? Thanks!

I made a small node wrapper for these. Thanks for providing them!

@ghost +1

I really need XRegExp.splitLb

I am terrible in regex. I have the following piece of code which is struggling with the look behind issue. can anyone help me solve it with provided code in the repo?

myString.replace(/\B(?<!\.\d*)(?=(\d{3})+(?!\d))/g,",")}

Is there a version to extend XRegExp instead of providing new methods? This suggestion is great but it doesn't polyfill the negative lookbehind, more like provides a standalone API to use it.