Regex works on RegExpr but not in Javascript. Matching between underscore and right before a forward slash

Question

I need to match all the stuff between an underscore _ and a backslash . So anything that's like bahbah_12345_12345\bahbah I want just the part that's _12345_12345.

I used regexr and regex101 to help me build what I want and looks like it works on, but when I put the same string and regular expression in my JS code it finds nothing.

For example here's my regex and sample string

Regex

_(.*?)(?=\\)

Sample String

IShouldGet3Match,allTheSequenceBetweenaUnderScoreAndASlashHeresOne:_1234_1234\andheresAnother_1234_1234_1234\OhLookAnotherOne!_123_12_1234567\bahbahba

On regex I get 3 matches, looks good. Then I put it in JS code and the regex doesn't match anything.

var pattern = /_(.*?)(?=\\)/g;
var result = str.match(pattern);

Here's the regexr and fiddle.

What did I do wrong?

If you print the source string, you'll see there are no backslashes in it. You forgot to escape the backslashes in the string literal, so \a is treated as a Bel character, \b becomes a backspace, and \O is just O. — Alan Moore
– Alan Moore, Commented Nov 14, 2015 at 5:39
Ohhh I get it now... feels convoluted to edit the string to escape all the backslash just to get my regex to work. I guess in cases like these you would just have to avoid looking using the backslash as part of your regex and build it some other way? — rawr rang
– rawr rang, Commented Nov 14, 2015 at 7:08
If I could offer an optimization to using .*?, try this: _([^\\]*)(?=\\) It will look for all characters after a _ until it hits the backslash. If you think you will encounter newlines that you wish to avoid, you can always add that to the character group: _([^\n\\]*)(?=\\) example 1. This takes 73 steps to find 9 matches. Otherwise, .*? will take 937 steps example 2. — OnlineCop
– OnlineCop, Commented Nov 14, 2015 at 8:17

Wiktor Stribiżew · Accepted Answer · 2015-11-14 09:11:12Z

I need to match all the stuff between an underscore _ and a backslash.

Your regex /_(.*?)(?=\\)/g does not really do that as it will stumble on a newline.

To actually match anything between an underscore and a backslash, you need a negated character class [^\\] - any character but a backslash.

Here:

/_([^\\]*)/g

This regex matches:

_ - matches an underscore
([^\\]*) - 0 or more characters other than a backslash.

JS sample snippets:

var str = 'IShouldGet3Match,allTheSequenceBetweenaUnderScoreAndASlashHeresOne:_1234_1234\\andheresAnother_1234_1234_1234\\OhLookAnotherOne!_123_12_1234567\\bahbahba';

var testPattern = /123/g;
var testResult = str.match(testPattern);
document.getElementById("demo").innerHTML = testResult;

var pattern = /_([^\\]*)/g;
var result = str.match(pattern);
document.getElementById("demo2").innerHTML = result;

var pattern = /_([^\\]*)/g;
while ((m=pattern.exec(str)) !== null) {
  document.getElementById("demo3").innerHTML += m[1]+"<br/>";
}

<body>
    <p>Just making a basic regex works with 123:</p>
    <p id="demo"></p>
    <p>Result of actual Regex:</p>
    <p id="demo2"></p>
    <p>Captured texts of actual Regex:</p>
    <p id="demo3"></p>
</body>

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