How to dynamically create regex to use in .match Javascript?

Question

I need to dynamically create a regex to use in match function javascript. How would that be possible?

var p = "*|";
var s = "|*";
"*|1387461375|* hello *|sfa|* *|3135145|* test".match(/"p"(\d{3,})"s"/g)

this would be the right regex: /\*\|(\d{3,})\|\*/g

even if I add backslashes to p and s it doesn't work. Is it possible?

Just concatenate `p`, `"(\d{3,})"`, and `s` (escape `p` and `s` per http://stackoverflow.com/questions/3446170/escape-string-for-use-in-javascript-regex) — Slippery Pete, Sep 03 '14 at 14:11

score 26 · Accepted Answer · edited Jul 01 '20 at 16:00

26

RegExp is your friend:

var p = "\\*\\|", s = "\\|\\*"

var reg = new RegExp(p + '(\\d{3,})' + s, 'g')

"*|1387461375|* hello *|sfa|* *|3135145|* test".match(reg)

The key to making the dynamic regex global is to transform it into a RegExp object, and pass 'g' in as the second argument.

Working example.

edited Jul 01 '20 at 16:00

FrostyDog

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answered Sep 03 '14 at 14:08

Daryl Ginn

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beat me to it by about 15 seconds :). It ends up as a bad regex, but it matches what the op asked – wolffer-east Sep 03 '14 at 14:10
Although the `new` operator isn't required when creating instances of built-in objects, it's [still recommended.](http://stackoverflow.com/questions/2712878/javascript-regex-pattern-concatenate-with-variable#comment2737765_2712896) – theftprevention Sep 03 '14 at 14:23
SyntaxError: Invalid regular expression: /*|(d{3,})|*/: Nothing to repeat. I also tried escaping p and s without success – Alexandru R Sep 04 '14 at 09:56
1

@AlexandruRada Edited. When using `RegExp`, you need to double escape. – Daryl Ginn Sep 04 '14 at 12:28
right, I was only escaping and wasn't working. – Alexandru R Sep 04 '14 at 12:44

score 0 · Answer 2 · answered Sep 03 '14 at 14:14

You can construct a RegExp object using your variables first. Also remember to escape * and | while forming RegExp object:

var p = "*|";
var s = "|*";
var re = new RegExp(p.replace(/([*|])/g, '\\$1')
                 + "(\\d{3,})" + 
                 s.replace(/([*|])/g, '\\$1'), "g");

var m = "*|1387461375|* hello *|sfa|* *|3135145|* test".match(re);
console.log(m);

//=> ["*|1387461375|*", "*|3135145|*"]

How to dynamically create regex to use in .match Javascript?

2 Answers2