Regex-Groups in Javascript

Question

I have a problem using a Javascript-Regexp.

This is a very simplified regexp, which demonstrates my Problem:

(?:\s(\+\d\w*))|(\w+)

This regex should only match strings, that doesn't contain forbidden characters (everything that is no word-character).

The only exception is the Symbol +
A match is allowed to start with this symbol, if [0-9] is trailing. And a + must not appear within words (44+44 is not a valid match, but +4ad is)

In order to allow the + only at the beginning, I said that there must be a whitespace preceding. However, I don't want the whitespace to be part of the match.

I tested my regex with this tool: http://regex101.com/#javascript and the resultig matches look fine.

There are 2 Issues with that regexp:

If I use it in my JS-Code, the space is always part of the match
If +42 appears at the beginning of a line, it won't be matched

My Questions:

How should the regex look like?
Why does this regex add the space to the matches?

Here's my JS-Code:

var input =  "+5ad6  +5ad6 sd asd+as +we";
var regexp = /(?:\s(\+\d\w*))|(\w+)/g;
var tokens = input.match(regexp);
console.log(tokens);

The space: see stackoverflow.com/questions/7505762/… -- using a non-capturing group does not exclude it from the total match, only from adding it to individual matching groups. — Jongware
– Jongware, Commented May 11, 2014 at 12:27
@Jongware so is there a way to extract the token without the space? Unfortunately, JS doesn't support lookbehinds — maja
– maja, Commented May 11, 2014 at 12:45

Bergi · Accepted Answer · 2014-05-11 12:48:29Z

2

How should the regex look like?

You've got multiple choices to reach your goal:

It's fine as you have it. You might allow the string beginning in place of the whitespace as well, though. Just get the capturing groups (tokens[1], tokens[2]) out of it, which will not include the whitespace.
If you didn't use JavaScript, a lookbehind could help. Unfortunately it's not supported.
Require a non-word-boundary before the +, which would make every \w character before the + prevent the match:
```
/\B\+\d\w+|\w+/
```

Why does this regex add the space to the matches?

Because the regex does match the whitespace. It does not add the \s(\+\d\w+) to the captured groups, though.

answered May 11, 2014 at 12:48

Bergi

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7 Comments

maja Over a year ago

The regex can even be improved: (\B\+\d)?\w+

Bergi Over a year ago

Yes, or (?:\B\+\d)?\w+. I just tried not to make a large change to the original expression.

maja Over a year ago

I take it back - this simplification doesn't work, because it would force tokens starting with a + to have at least 2 characters afterwards

Bergi Over a year ago

But doesn't the original expression do the same? Or did you mean \+\d\w*

maja Over a year ago

Yes, I meant \+\d\w*

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