I am trying to find a regular expression that will match a string when it's NOT preceded by another specific string (in my case, when it is NOT preceded by "http://"). This is in JavaScript, and I'm running on Chrome (not that it should matter).

The sample code is:

var str = ' www.stackoverflow';
alert(str.replace(new RegExp('SOMETHING','g'),'rocks'));

And I want to replace SOMETHING with a regular expression that means "match www.stackoverflow unless it's preceded by http://". The alert should then say " rocks", naturally.

Can anyone help? It feels like I tried everything found in previous answers, but nothing works. Thanks!

I am trying to find a regular expression that will match a string when it's NOT preceded by another specific string (in my case, when it is NOT preceded by "http://"). This is in JavaScript, and I'm running on Chrome (not that it should matter).

The sample code is:

var str = 'http://www.stackoverflow. www.stackoverflow.';
alert(str.replace(new RegExp('SOMETHING','g'),'rocks'));

And I want to replace SOMETHING with a regular expression that means "match www.stackoverflow. unless it's preceded by http://". The alert should then say "http://www.stackoverflow. rocks", naturally.

Can anyone help? It feels like I tried everything found in previous answers, but nothing works. Thanks!

• javascript
• regex

Share Improve this question Follow asked Sep 18, 2013 at 23:57 Assaf HershkoAssaf Hershko 1,93444 gold badges1818 silver badges2020 bronze badges 10

• 1 Read about "negative lookaheads". – elclanrs Commented Sep 18, 2013 at 23:59
• Techically, this whole string is not preceded by http://; should it be replaced as a whole? – raina77ow Commented Sep 19, 2013 at 0:03
• Uncaught SyntaxError: Invalid regular expression: /(?<!http:\/\/)www\.stackoverflow\./: Invalid group – Assaf Hershko Commented Sep 19, 2013 at 0:03
• 2 JS regex engine doesn't support lookbehinds. – raina77ow Commented Sep 19, 2013 at 0:05
• 1 All I'm looking for is a RegEx that allows me to match string XXX if it's not preceded by YYY, in JavaScript. – Assaf Hershko Commented Sep 19, 2013 at 0:09

 |  Show 5 more ments

3 Answers 3

Sorted by: Reset to default 4

As JavaScript regex engines don't support 'lookbehind' assertions, it's not possible to do with plain regex. Still, there's a workaround, involving replace callback function:

var str = "As http://JavaScript regex engines don't support `lookbehind`, it's not possible to do with plain regex. Still, there's a workaround";

var adjusted = str.replace(/\S+/g, function(match) {
  return match.slice(0, 7) === 'http://'
    ? match
    : 'rocks'
});
console.log(adjusted);

You can actually create a generator for these functions:

var replaceIfNotPrecededBy = function(notPrecededBy, replacement) {
   return function(match) {
     return match.slice(0, notPrecededBy.length) === notPrecededBy
       ? match
       : replacement;
   }
};

... then use it in that replace instead:

var adjusted = str.replace(/\S+/g, replaceIfNotPrecededBy('http://', 'rocks'));

JS Fiddle.

raina77ow's answer reflected the situation in 2013, but it is now outdated, as the proposal for lookbehind assertions got accepted into the ECMAScript spec in 2018.

See docs for it on MDN:

Characters	Meaning
(?<!y)x	Negative lookbehind assertion: Matches "x" only if "x" is not preceded by "y". For example, /(?<!-)\d+/ matches a number only if it is not preceded by a minus sign. /(?<!-)\d+/.exec('3') matches "3". /(?<!-)\d+/.exec('-3') match is not found because the number is preceded by the minus sign.

Therefore, you can now express "match www.stackoverflow. unless it's preceded by http://" as /(?<!http:\/\/)www.stackoverflow./:

const str = 'http://www.stackoverflow. www.stackoverflow.';
console.log(str.replace(/(?<!http:\/\/)www.stackoverflow./g, 'rocks'));

This also works:

var variable = 'http://www.example. www.example.';
alert(variable.replace(new RegExp('([^(http:\/\/)|(https:\/\/)])(www.example.)','g'),'$1rocks'));

The alert says "http://www.example. rocks".