Convert string to RegExp

Question

How can I check a JavaScript string is in a RegExp format, then convert it to RegExp?

I found a way with RegExp, but the rule is too complex to make it right.

function str2Regex(str){
  var rule = /\/(\\[^\x00-\x1f]|\[(\\[^\x00-\x1f]|[^\x00-\x1f\\\/])*\]|[^\x00-\x1f\\\/\[])+\/([gim]*)/;
  var match = str.match(rule);
  return match ? new RegExp(match[1],match[3]) : str;
}

Now I'm using /\/(.*)\/(?=[igm]*)([igm]*)/ which works.

Why do you need the check the validity of a regular expression? That seems like a serious issue. — Halcyon
– Halcyon, Commented Jan 8, 2013 at 17:07
I'm writing some route function, use a cfg object, which key is string or regex, value is handlerFn, because the object key must be a string, so I had to detect the key is a string or regex string, then change it to a Regex — atian25
– atian25, Commented Jan 9, 2013 at 3:53
I've seen your edit. Your solution fails on something as simple as /**/i. You really shouldn't try to guess if a string is correct as a regex, just let the browser finds it (i.e. use try/catch around new RegExp). — Denys Séguret
– Denys Séguret, Commented Jan 9, 2013 at 11:27
Another problem with your approach is that regex engines change. Supposing it's possible to find a regex validing regexes, it wouldn't be the same when new regex feature are introduced in javascript regex engines. — Denys Séguret
– Denys Séguret, Commented Jan 9, 2013 at 11:33

Denys Séguret · Accepted Answer · 2013-01-09 07:04:49Z

7

The simplest way, and probably the most correct, is to use try/catch :

try {
 r = new RegExp(str);
} catch(error) {
  // no good
}

You get a SyntaxError when the string doesn't match a well formed regular expression.

If you want to test a string whose value is like a compiled regular expression (for example "/\b=\b/g", you can use such a function :

function checkCompiledRegex(str) {
  if (str[0]!='/') return false;
  var i = str.lastIndexOf('/');
  if (i<=0) return false;
  try {
    new RegExp(str.slice(1, i), str.slice(i+1));
  } catch(error) {
    return false;
  }
  return true;
}

edited Jan 9, 2013 at 7:04

answered Jan 8, 2013 at 17:08

Denys Séguret

384k90 gold badges813 silver badges780 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

atian25 Over a year ago

RegExp's param seems can't use / and igm. such as: ` "y".match(new RegExp("/y/i")) ` is null.

Denys Séguret Over a year ago

When you build a regex from a string, remove the "/" and the modifiers. /y/i is a compiled regex.

atian25 Over a year ago

I know RegExp("\d+","igm"), so I'm searching for a regex to extract them from string "/\d+/igm"

atian25 Over a year ago

if the regexp has a '/', this will not pass. for example, we can use /(w+)\/(\d+)/ to detect match "a/2" , then the regex string is "/(w+)\\/(\d+)/" which can't pass your fn

Collectives™ on Stack Overflow

Convert string to RegExp

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related