Javascript: Convert a String to Regular Expression

Question

I want to convert a string that looks like a regular expression...into a regular expression.

The reason I want to do this is because I am dynamically building a list of keywords to be used in a regular expression. For example, with file extensions I would be supplying a list of acceptable extensions that I want to include in the regex.

var extList = ['jpg','gif','jpg'];

var exp = /^.*\.(extList)$/;

Thanks, any help is appreciated

ChaosPandion · Accepted Answer · 2011-01-13 04:28:48Z

9

You'll want to use the RegExp constructor:

var extList = ['jpg','gif','jpg'];    
var reg = new RegExp('^.*\\.(' + extList.join('|') + ')$', 'i');

MDC - RegExp

answered Jan 13, 2011 at 4:28

ChaosPandion

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3 Comments

Sarwar Erfan Over a year ago

Another way can be to use eval(...), but creating Regexp object is far way better solution.

Phrogz Over a year ago

a) RegExp, not Regexp; b) You created /^.*.(jpg|gif|jpg)$/ (note the missing backslash); c) The third parameter is optional if you aren't going to pass any flags.

Embedded_Mugs Over a year ago

I noticed that this method does't accept regex's that start and end with a slash, ie "/^b/". is there a way to account for this besides manually trimming the slashes. (The string coms from user input).

Phrogz · Accepted Answer · 2011-01-13 04:40:28Z

2

var extList = "jpg gif png".split(' ');
var exp = new RegExp( "\\.(?:"+extList.join("|")+")$", "i" );

Note that:

You need to double-escape backslashes (once for the string, once for the regexp)
You can supply flags to the regex (such as case-insensitive) as strings
You don't need to anchor your particular regex to the start of the string, right?
I turned your parens into a non-capturing group, (?:...), under the assumption that you don't need to capture what the extension is.

Oh, and your original list of extensions contains 'jpg' twice :)

edited Jan 13, 2011 at 4:40

answered Jan 13, 2011 at 4:30

Phrogz

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Comments

Chandu · Accepted Answer · 2011-01-13 04:31:50Z

1

You can use the RegExp object:

var extList = ['jpg','gif','jpg'];

var exp = new RegExp("^.*\\.(" + extList.join("|") + ")$");

answered Jan 13, 2011 at 4:31

Chandu

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