RegExp differs from using a simple regex literal

Question

I am trying to lift properties from a string like this:

var valueAccessor = "function (){return { id:'someId',label:'Some Label',value:PropertyOnModel} }";

This works when I specifically want to get value from the above string, specifically using this code:

var value = valueAccessor.match(/function(?=[^}]*value\s?:\s?(\w+))/im)[1]

I have a list of the different properties I want to lift, for example label and id so I iterate through them and dynamically set my regex using RegExp.

// Iterate through id, label, value
for(var prop in obj) {
 var propRegex = new RegExp('/function(?=[^}]*' + prop + '\\s?:\\s?(\\w+))/im');
 var propValue = valueAccessor.match(propRegex)[1];
}

Although the propRegex source is identical to the structure in the first example, it returns undefined and thus means I can't extract the relative values.

Can anybody explain the difference between the two and where I am going wrong?

The / are the delimiters of the regex literal, just like " are the delimiters of a string. They tell the parser where the pattern starts and ends. They are not part of the value (pattern) itself. If you don't use a regex literal, don't put / around the pattern. — Felix Kling
– Felix Kling, Commented Sep 3, 2014 at 15:09

epascarello · Accepted Answer · 2014-09-03 15:07:26Z

3

It is

new RegExp(pattern, flags);

so you need to drop the / and move the im out

var propRegex = new RegExp('function(?=[^}]*' + prop + '\\s?:\\s?(\\w+))', 'im');

Reference: MDN RegExp constructor

answered Sep 3, 2014 at 15:07

epascarello

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4 Comments

LukeHennerley Over a year ago

Will accept the answer in a second, I haven't really used Regex to a high level in Javascript so I think I got a bit confused because I didn't specify any flags in the original string.match (even though there is an overload)

LukeHennerley Over a year ago

Felix Kling's comment has just kicked in... I understand now. When Stack lets me accept I will, cheers for that - had me caught for a while now! :P

Brian Stephens Over a year ago

The regex is also needlessly complicated, even though it works. There's no need for the lookahead: 'function[^}]*' + prop + '\\s?:\\s?(\\w+)'. Perhaps he meant to use a non-capturing group instead (?: ).

LukeHennerley Over a year ago

@BrianStephens I despise Regex! I spoke to one of our "regex guru's" in the office for this, I am sure he will be pleased to have your feedback ;)

Mindastic · Accepted Answer · 2014-09-03 15:08:09Z

0

It seems that your RegExp below is not well formed.

Try doing:

for(var prop in obj) {
 var propRegex = new RegExp("function(?=[^}]*" + prop + "\\s?:\\s?(\\w+))", "im");
 var propValue = valueAccessor.match(propRegex)[1];
}

answered Sep 3, 2014 at 15:08

Mindastic

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3 Comments

Felix Kling Over a year ago

That would literally look for + prop + inside the string, which is certainly not what the OP wants.

Mindastic Over a year ago

I forgot to change the single quotes inside the RegExp. I changed the first and last one (cause i prefer using double quotes) but forgot to change the others.

Felix Kling Over a year ago

That's why it's always good to properly explain the problem and solution ;) Otherwise it's hard to tell what you changed to solve the problem and what might just be a typo.

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