Weird javascript regexp replace issue

Question

I have the following text

text(-) 3

Which I want to change into:

-3

So I wrote the following regular expression:

'text(-) 3'.replace(/text\((?:[^\)]+)\)/g, RegExp.$1);

DEMO - which doesn't work.

However, when I do the following

var output = 'text(-) 3'.replace(/text\(([^\)]+)\)/g, RegExp.$1);
output = 'text(-) 3'.replace(/text\((?:[^\)]+)\)/g, RegExp.$1);

(So in the first I've removed the ?:) it works

DEMO

So there must be something I've missed with grouping matches with regular expressions (or just something stupid). Any help would be appreciated!?

I can't, because in my case other text can be present in the string which I don't want to replace and I really would like to know whats going on here! — Jeanluca Scaljeri
– Jeanluca Scaljeri, Commented Dec 15, 2014 at 15:01

nhahtdh · Accepted Answer · 2014-12-15 15:27:00Z

Code explanation

Removing ?: makes the group capturing, which means that it will capture anything matched by [^\)]+, and the matched text is made available for replacement and backreference.

var output = 'text(-) 3'.replace(/text\(([^\)]+)\)/g, RegExp.$1);

When String.replace is called, RegExp.$1 is not yet initialized, so the result in output will be " 3". Then the regex is executed, and the capturing group captures - in text(-) and place it in capturing group 1.

The deprecated attribute RegExp.$1 is also updated with the content of the capturing group 1 of the last RegExp execution.

Then on the next line:

output = 'text(-) 3'.replace(/text\((?:[^\)]+)\)/g, RegExp.$1);

Since RegExp.$1 now contains "-", the replacement give the result "- 3".

RegExp.$1 is deprecated, since it is shared between all execution of RegExp and extremely error prone. So are a bunch of other properties RegExp.$2 to RegExp.$9 which contains the content of the capturing group 2 to 9 of the latest execution.

Solution

For your purpose, you should do this:

var output = 'text(-) 3'.replace(/text\(([^\)]+)\) */g, "$1");

It is not clear how many spaces after ), so I added *. To refer to the content of a capturing group in the replacement, use $1 in the replacement string.

nu11p01n73R · Accepted Answer · 2014-12-15 15:21:17Z

1

You might be looking for

var output = 'text(-) 3'.replace(/text\(([^\)]+)\)/g, '$1');

Fiddle Demo

Problems with 'text(-) 3'.replace(/text$(?:[^$]+)\)/g, RegExp.$1);

(?:[^\)]+) is a non capturing group so the value is not captured in $1
RegExp.$1 Contains the capture group from the previous regex match and not the current

That is
```
var output = 'text(-) 3'.replace(/text$([^$]+)\)/g, RegExp.$1);
output = 'text(-) 3'.replace(/text$(?:[^$]+)\)/g, RegExp.$1);
```
Here in the second statement the regex RegExp.$1 is from the first statement and not from the second. So when used alone it is unset and does not create the expected output.

edited Dec 15, 2014 at 15:21

answered Dec 15, 2014 at 15:15

nu11p01n73R

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