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);

Answer 1

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.