Regex optional group

Question

I am using this regex:

((?:[a-z][a-z]+))_(\d+)_((?:[a-z][a-z]+)\d+)_(\d{13})

to match strings like this:

SH_6208069141055_BC000388_20110412101855

separating into 4 groups:

SH
6208069141055
BC000388
20110412101855

Question: How do I make the first group optional, so that the resulting group is a empty string?
I want to get 4 groups in every case, when possible.

Input string for this case: (no underline after the first group)

6208069141055_BC000388_20110412101855

score 212 · Answer 1 · answered Dec 18 '19 at 11:44

212

Making a non-capturing, zero to more matching group, you must append ?.

(?: ..... )?
^          ^____ optional
|____ group

answered Dec 18 '19 at 11:44

Daniel W.

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what is the difference between option group and (...)* – Golden Lion Nov 09 '21 at 21:50
1

@GoldenLion `?` matches "zero or one times" and `*` matches "zero or many times" – Daniel W. Nov 10 '21 at 09:26
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What does the ```?:``` at the start of the group do? – brobers Aug 19 '22 at 21:59
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@brobers a normal group `(...)` will capture the content in the results, but `(?:...)` makes it a non-capturing group, so it must match (or apply) but will not be included in the resulting groups. – Daniel W. Aug 21 '22 at 03:48

Jerry · Accepted Answer · 2020-07-24T16:20:49.123

97

You can easily simplify your regex to be this:

(?:([a-z]{2,})_)?(\d+)_([a-z]{2,}\d+)_(\d+)$
^              ^^
|--------------||
| first group  ||- quantifier for 0 or 1 time (essentially making it optional)

I'm not sure whether the input string without the first group will have the underscore or not, but you can use the above regex if it's the whole string.

regex101 demo

As you can see, the matched group 1 in the second match is empty and starts at matched group 2.

edited Jul 24 '20 at 16:20

answered Sep 05 '13 at 10:08

Jerry

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Which of the characters is marking the group optional? – Daniel W. Dec 18 '19 at 11:34
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The `?` after the first group? That's the 17th character from the left. – Jerry Dec 18 '19 at 16:30

Regex optional group

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