Unexpected behavior of regexp in JavaScript

Question

I've encountered this weird behavior: enter image description here

I'm on a breakpoint (variables don't change). At the console you can see, that each time I try to evaluate regexp methods on the same unchanging variable "text" I get these opposite responses. Is there an explanation for such thing?

The relevant code is here:

this.singleRe = /<\$([\s\S]*?)>/g;    

while( this.singleRe.test( text ) ){
        match = this.singleRe.exec( text );

        result = "";

        if( match ){

            result = match[ 1 ].indexOf( "." ) != -1 ? eval( "obj." +  match[ 1 ] ) :  eval( "value." + match[ 1 ] );

        }

        text = text.replace( this.singleRe , result );

    }

Could you please post your code including the regular expression being used and the text that is being matched? Unclear what is causing this from the provided screenshot... — MaineCoder
– MaineCoder, Commented Jan 9, 2014 at 12:39
could you please share your code and which browser you are using with version. — Abhishek Shukla
– Abhishek Shukla, Commented Jan 9, 2014 at 12:40

georg · Accepted Answer · 2014-01-09 13:11:33Z

4

When you use regex with exec() and a global flag - g, a cursor is changing each time, like here:

var re = /\w/g;
var s = 'Hello regex world!'

re.exec(s); // => ['H']
re.exec(s); // => ['e']
re.exec(s); // => ['l']
re.exec(s); // => ['l']
re.exec(s); // => ['o']

Note the g flag! This means that regex will match multiple occurencies instead of one!

EDIT

I suggest instead of using regex.exec(string) to use string.match(regex) if possible. This will yield an array of occurences and it is easy to inspect the array or to iterate through it.

edited Jan 9, 2014 at 13:11

georg

216k57 gold badges324 silver badges401 bronze badges

answered Jan 9, 2014 at 12:40

lukas.pukenis

13.7k13 gold badges50 silver badges83 bronze badges

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

3 Comments

Michael Sazonov Over a year ago

What a wonderful and simple explanation. It works! Thank you very much.

lukas.pukenis Over a year ago

When you do .exec() you can do string.match(regex) which will return you the whole array of occurences so you will be able to iterate through the array and inspect more easily :)

tenub Over a year ago

You could also do it like this: var res; while (res = re.exec(s)) { // do something with each result, maybe add to an array }.

Collectives™ on Stack Overflow

Unexpected behavior of regexp in JavaScript

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related