Regex lookahead. Find word without .min. in string

Question

I'm trying to replace a link in a html file with regex and nodejs. I want to replace links without a .min.js extension.

For example, it should match "common.js" but not "common.min.js"

Here's what I've tried:

let htmlOutput = html.replace(/common\.(?!min)*js/g, common.name);

I think this negative lookahead should work but it doesn't match anything. Any help would be appreciated.

Maybe just `let htmlOutput = html.replace(/\bcommon\.js\b/g, common.name);`? `(?!min)*js` makes no sense. — Wiktor Stribiżew, Jun 05 '19 at 13:47
It [works](https://regex101.com/r/zf6RHX/1). Also, see https://jsfiddle.net/wxvL9m7p/ — Wiktor Stribiżew, Jun 05 '19 at 13:55
@WiktorStribiżew Confusing. It must be another issue preventing a match. I'll take a look and get back to you. — Ian Gleeson, Jun 05 '19 at 13:57
It's from WebPack. A string containing the js filename with a generated contenthash for cache busting — Ian Gleeson, Jun 05 '19 at 13:59
Ok, if it can contain `$`, use `common.name.replace(/\$/g, '$$$$')`. But you should check what you are doing because the regex I shared must work. — Wiktor Stribiżew, Jun 05 '19 at 14:02
Your regex is correct but it still doesn't replace the string in my html. Not sure why. I'll do some tests — Ian Gleeson, Jun 05 '19 at 14:07
Are you sure you are assigning the new value after replacement to the variable? — Wiktor Stribiżew, Jun 05 '19 at 14:12
Turns out is was an assignment issue with my file variable. Works perfectly now. Thank you so much — Ian Gleeson, Jun 05 '19 at 14:14
Will somebody please formulate a proper answer, please? :) So we can up-vote... — virolino, Jun 05 '19 at 14:25
@WiktorStribiżew Sorry, could I ask for help again? The regex doesn't fully fufill my use case. Searching for the word 'common.js' will work the first time. How do I search the second time when the string will look like 'common.9132423487928.js'? I tried .* which predictably brought me common.345345345.js and common.456456456.min.js — Ian Gleeson, Jun 05 '19 at 14:35
Found it. I'll post an answer below crediting you @WiktorStribiżew — Ian Gleeson, Jun 05 '19 at 14:58
@virolino I posted the right [answer](https://stackoverflow.com/a/56464824/3832970). — Wiktor Stribiżew, Jun 05 '19 at 16:54

Wiktor Stribiżew · Accepted Answer · 2019-06-05T17:39:49.367

The (?!min)*js part is corrupt: you should not quantify zero-width assertions like lookaheads (they do not consume text so quantifiers after them are treated either as user errors or are ignored). Since js does not start with min this lookahead even without a quantifier is redundant.

If you want to match a string with a whole word common, then having any chars and ending with .js but not .min.js you need

/\bcommon\b(?!.*\.min\.js$).*\.js$/

See the regex demo.

Details:

\b - word boundary
common - a substring
\b - word boundary
(?!.*\.min\.js$) - immediately to the right, there should not be any 0 or more chars followed with .min.js at the end of the string
.* - any 0 or more chars
\.js - a .js substring
$ - end of string.

score 1 · Answer 2 · answered Jun 05 '19 at 15:12

1

Here, we likely can find a simple expression to pass any char except new lines and ., after the word common, followed by .js:

common([^\.]+)?\.js

Demo

RegEx Circuit

jex.im visualizes regular expressions:

answered Jun 05 '19 at 15:12

Emma

27,428
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score 0 · Answer 3 · answered Jun 05 '19 at 15:05

0

The end regex I'm using is /\bcommon[^min]+js\b/g

This will find the word common with any amount of chracters afterword except if those characters contain the word minand ending in js allowing me to replace scripts on my html page like:

script src="~/dist/common.js"

OR

script src="~/dist/common.9cf5748e0e7fc2928a07.js"

Thanks to Wiktor Stribiżew for helping me.

answered Jun 05 '19 at 15:05

Ian Gleeson

383
3
17

You are rather wrong. `\bcommon[^min]+js\b` matches `common` followed with 1 or more chars other than `m`, `i`, `n` followed with `js` substring. So you need `\bcommon(?:(?!min).)*?js\b` I guess, judging by your description. Or `\bcommon\b(?!\S*\.min\.js\b)\S*\.js\b` – Wiktor Stribiżew Jun 05 '19 at 16:37

Regex lookahead. Find word without .min. in string

3 Answers3

Demo

RegEx Circuit