How to prevent regex ^(a)?$ from capturing empty line?

Question

My goal is to regex a multiline string. The problem is that it captures empty line.

The regex: ^(a)?$

The string:

The result:

Match 1 a
Group 1 a
Match 2 null
Group 1 null

If `a` is optional between start and end anchors then by design it will allow empty string. What is your requirement? — anubhava, Aug 26 '22 at 18:25
@anubhava Noted. My requirements are `regex that can group a stringified table which have empty data on several row, column`. [Here is the previous SO's question that contain the full requirements](https://stackoverflow.com/questions/73502078/is-it-possible-for-regex-to-recognize-whether-a-column-is-a-string-or-an-int/73502188#73502188). Anyway, is there an alternative quantifier? — Jason Rich Darmawan, Aug 26 '22 at 18:33
@anubhava it throws error "the preceding token is not quantifiable" — Jason Rich Darmawan, Aug 26 '22 at 18:41
@anubhava sorry my bad, Golang use RE2, negative lookahead operator is not supported. https://stackoverflow.com/questions/47211017/regex-expression-negated-set-not-working-golang — Jason Rich Darmawan, Aug 26 '22 at 18:46
That is as per the spec. You want to match & capture `a` or else match 1+ char — anubhava, Aug 26 '22 at 18:56
@anubhava It works. I modify the first line to `abc` and modify the regex to `^(?:([a-z]+)|.+)$`. Thank you. — Jason Rich Darmawan, Aug 26 '22 at 19:00
@anubhava after double checking the capturing group `([a-z]+)` should have quantifier, because the requirements are `table which can have empty data on several row, column` -> `([a-z]+)?`. If you add that, it will capture the empty line again. — Jason Rich Darmawan, Aug 26 '22 at 19:04
No you don't add `?` because your desired match is on LHS of `|` and is available in capture group. On RHS is 1+ of any char not in any capture group that is outside your match — anubhava, Aug 26 '22 at 19:06

score 1 · Accepted Answer · answered Aug 26 '22 at 19:02

Converting my comment to answer so that solution is easy to find for future visitors.

You may use this regex in golang to prevent this behavior:

^(?:([a-z]+)|.+)$

This will match & capture 1+ of lowercase characters or else match any 1+ char but don't allow empty match.

RegEx Demo

How to prevent regex ^(a)?$ from capturing empty line?

1 Answers1