How to dynamically create regex to use in .match Javascript? [duplicate]

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 stackoverflow.com/questions/3446170/…) — Slippery Pete
– Slippery Pete, Commented Sep 3, 2014 at 14:11

TheScrappyDev · Accepted Answer · 2020-07-01 16:00:46Z

34

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 1, 2020 at 16:00

TheScrappyDev

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answered Sep 3, 2014 at 14:08

Daryl Ginn

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5 Comments

wolffer-east Over a year ago

beat me to it by about 15 seconds :). It ends up as a bad regex, but it matches what the op asked

theftprevention Over a year ago

Although the new operator isn't required when creating instances of built-in objects, it's still recommended.

Alexandru R Over a year ago

SyntaxError: Invalid regular expression: /*|(d{3,})|*/: Nothing to repeat. I also tried escaping p and s without success

Daryl Ginn Over a year ago

@AlexandruRada Edited. When using RegExp, you need to double escape.

Alexandru R Over a year ago

right, I was only escaping and wasn't working.

anubhava · Accepted Answer · 2014-09-03 14:14:45Z

0

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|*"]

answered Sep 3, 2014 at 14:14

anubhava

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