JavaScript - replace substring using a variable

Question

var prefix = 'pre-',
    number = 1,
    combined = prefix + number,
    prefixRemoved = combined.replace('/' + prefix + '/g', '');
console.debug(prefixRemoved);

How do I remove prefix from combined? I'm still getting pre-1 as a result.

possible duplicate of How do you pass a variable to a Regular Expression JavaScript? — user113716
– user113716, Commented Sep 21, 2011 at 2:15

jfriend00 · Accepted Answer · 2011-09-21 02:05:41Z

1

You can just pass a string to replace() like this:

prefixRemoved = combined.replace(prefix, '');

or if you need parameters on the regular expression, you can create a regexp from a string like this:

prefixRemoved = combined.replace(new RegExp(prefix, "g"), '');

or, you can create the regex object first:

var re = new RegExp(prefix, "g");
prefixRemoved = combined.replace(re, '');

answered Sep 21, 2011 at 2:05

jfriend00

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Comments

Joe · Accepted Answer · 2011-09-21 02:15:21Z

1

You need to create the RegExp object:

var prefix = 'pre-',
    number = 1,
    combined = prefix + number,
    prefixRemoved = combined.replace(new RegExp(prefix, 'g'), '');
console.debug(prefixRemoved);

Example

Why that is happening:

// this syntax without quotes is shorthand for creating a RegExp object
typeof /aaaa/; // object
/a/ instanceof RegExp; // true

// since String.replace can take a string or a RegExp, it has to assume that any string is just a string 
typeof "/aaaa/"; // string

edited Sep 21, 2011 at 2:15

answered Sep 21, 2011 at 2:05

Joe

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Comments

Joseph Silber · Accepted Answer · 2011-09-21 02:05:24Z

0

You don't need a regex for this. Simply use the string as is:

var prefix = 'pre-',
    number = 1,
    combined = prefix + number,
    prefixRemoved = combined.replace(prefix, '');
console.debug(prefixRemoved);

answered Sep 21, 2011 at 2:05

Joseph Silber

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6 Comments

Joe Over a year ago

+1, but I guess, techically it can be simplified to console.log(number)

zzzzBov Over a year ago

there is a difference between /prefix/g and prefix, one replaces one match, the other replaces all matches. In this instance it might not be an issue, but the general question is how to use a variable within a RegEx.

Joseph Silber Over a year ago

@zzzzBov - I was simply trying to help the OP solve his problem. A lot of people jump first to regex when they have a problem. But really, that should be the last tool you use, after everything else in your toolkit has failed you...

zzzzBov Over a year ago

a global variable replace is very common. I'd rather have people understand how to make a simple RegEx to do what they need rather than avoid RegEx out of fear of the unknown.

Joseph Silber Over a year ago

@zzzzBov - He specifically asked to remove a prefix, and the best way to do that is the way I've shown him...

|

Jason McCreary · Accepted Answer · 2011-09-21 02:05:54Z

0

There's really no need for using regular expressions since you are merely performing string replacement.

prefixRemoved = combined.replace(prefix, '');

See it in action: http://jsfiddle.net/jjYC8/

answered Sep 21, 2011 at 2:05

Jason McCreary

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3 Comments

zzzzBov Over a year ago

this is incorrect, it will only replace the first occurrence of prefix. Using /prefix/g will replace all occurrences.

Jason McCreary Over a year ago

True. But look at the example code: pre-1. So there's no need for the global replacement. Thanks for the down vote though.

Jason Gennaro Over a year ago

+1 there is no need to vote this down @zzzzBov. The OP never said he needed to replace more than one instance.

Jason Gennaro · Accepted Answer · 2011-09-21 02:08:04Z

0

You need prefixRemoved = combined.replace(prefix, '');

var prefix = 'pre-',
    number = 1,
    combined = prefix + number,
    prefixRemoved = combined.replace(prefix, '');
console.debug(prefixRemoved);

Just call the variable directly.

Example: http://jsfiddle.net/jaffc/

answered Sep 21, 2011 at 2:08

Jason Gennaro

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