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@slevithan
Created April 14, 2012 21:06
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Simulating lookbehind in JavaScript (take 2)
// 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"
@slevithan
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Author

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

@JoeWarwick
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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 !>.

@JoeWarwick
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sorry. to continue :

e.g.

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

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

Is this possible?

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

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ghost commented Apr 10, 2015

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

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

@mpyw
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mpyw commented Sep 27, 2018

@ghost +1

I really need XRegExp.splitLb

@byteab
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byteab commented Apr 26, 2021

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,",")}

@kettanaito
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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.

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