Regular expression to match any number unless it's followed by a given character-CodePudding

I'm stuck on creating a reg ex that matches and groups any number of digits unless it's followed directly by a letter A

What I want:

1     -> 1
1B    -> 1
11B   -> 11
1A    -> nothing
11A   -> nothing

In the reg ex I have (https://regex101.com/r/jdlDe9/1), I use a negative look-ahead: .*(\d )(?!A). It works if a single digit is followed by an A but still matches if multiple digits are followed by an A

1     -> 1        OK
1B    -> 1        OK
12B   -> 12       OK
1A    -> nothing  OK
12A   -> 1        NOT OK. Here it matches the first `1` but I don't want to match anything

CodePudding user response：

The quantifiers in the pattern .*(\d )(?!A) allow backtracking, so the the \d that matches 12 in 12A can backtrack one position and have the lookahead evaluate to true.

You can match capture 1 digits, followed by matching any char except A or a digit, or assert the end of the string

(\d )(?:[^A\d]|$)

Regex demo

Or if possessive quantifiers are supported, that not allow backtracking after there is a match, and you can get a match only without the capture group:

\d  (?!A)

Regex demo