Regex difference: (\w+)? and (\w*)

Question

Is there any difference between (\w+)? and (\w*) in regex?

It seems the same, doesn't it?

It does seem the same, except if you care about `""` vs. `null` — John Dvorak, Jan 17 '13 at 06:31
`(\w+)?` seems odd. Where did you see that? Any link to external resource please? — Rohit Jain, Jan 17 '13 at 06:33

John Dvorak · Accepted Answer · 2013-01-17T06:53:08.690

34

(\w+)? and (\w*) both match the same (0..+inf word characters)

However, there is a slight difference:

In the first case, if this part of the regex matches "", the capturing group is absent. In the second case, it is empty. In some languages, the former manifests as a null while the latter should always be "".

In Javascript, for example,

/(\w*)/.exec("")  // ["", ""]
/(\w+)?/.exec("") // ["", undefined]

In PHP (preg_match), in the former case, the corresponding key is simply absent in the matches array: http://3v4l.org/DB6p3#v430

edited Jan 17 '13 at 06:53

answered Jan 17 '13 at 06:37

John Dvorak

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In which language the capturing of `""` results in **null** or empty string ? – Cozzamara Jan 17 '13 at 06:40
@Cozzamara In the first case, an empty match is _not_ captured. – John Dvorak Jan 17 '13 at 06:41
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By which engine ? Both Perl and SED **do** capture empty string by both patterns – Cozzamara Jan 17 '13 at 06:46
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Thanks, I never realized that non-matched groups result in `undefined` instead of empty strings (since I always only check for truthiness) – Bergi Jan 17 '13 at 07:44

Regex difference: (\w+)? and (\w*)

1 Answers1