I'm sorry for asking this maybe duplicate question. I checked the existing questions and answers about optional capturing groups. I tried some things but I'm not able to translate the answer to my own example.

This are two imput lines

id:target][label
id:target

I would like to capture id: (group 1), target (group 2) and if ][ is present label (group 3).

The used regex (Python regex) only works on the first line (

In the other examples I don't get what the regex makes a capturing group optional. And maybe the delimiter ][ used by me also mix up with my understanding problem.

One thing I tried was this

^(. :)(.*)(\]\[(.*))?

This doesn't work as expected

CodePudding user response：

You could write the pattern using an anchor at the end, a negated character class for group 1, a non greedy quantifier for group 2 and then optionally match a 3rd part:

^([^:] :)(.*?)(?:]\[(.*))?$

Explanation

• ^ Start of string
• ([^:] :) Group 1, match 1 chars other than : and then match : using a negated character class
• (.*?) Group 2, match any char, as few as possible
• (?: Non capture group to match as a whole part
  • ]\[ Match ][
  • (.*) Group 3, match any character
• )? Close the non capture group and make it optional
• $ End of string

See a regex101 demo

If you are only matching for example word characters, this you might consider:

^([^:] :)(\w )(?:]\[(\w ))?

See a another regex101 demo