Regular Expression - Match String Not Preceded by Another String (JavaScript)

Question

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

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

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

Answer 1

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 .

Answer 2

This also works:

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

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

Answer 3

raina77ow's answer reflected the situation in 2013, but it is now outdated, as the proposal for lookbehind assertions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

Characters<\/th>

Meaning<\/th><\/tr><\/thead>

(?<!y)x<\/code><\/td>

Negative lookbehind assertion:<\/strong> Matches "x" only if "x" is not preceded by "y". For example, \/(?<!-)\\d+\/<\/code> matches a number only if it is not preceded by a minus sign. \/(?<!-)\\d+\/.exec('3')<\/code> matches "3". \/(?<!-)\\d+\/.exec('-3')<\/code> match is not found because the number is preceded by the minus sign. <\/td><\/tr><\/tbody><\/table><\/blockquote> Therefore, you can now express "match www.stackoverflow.com<\/code> unless it's preceded by http:\/\/<\/code> " as \/(?<!http:\\\/\\\/)www.stackoverflow.com\/<\/code> :