I'm looking for a regular expression to match every characters that are between two a or two b but not between an a and a b.

For example:

accccca → should match
bfffffb → should match
azzzzzb → shouldn't match
bttttta → shouldn't match

ammbmma → should match from the a to the a
bllalla → should match only between the two a

How can I do that?

CodePudding user response：

I think something like a. a|b. b should work.

. means at least one character, so a. a would match an a, at least one but up to infinity other characters, followed by another a. b. b works similarly, and then | works to OR the the two expressions together.

CodePudding user response：

Try this pattern: (a|b)(.*)\1

See Regex Demo

Explanation

• (...) A captured group (group 1)
• a|b Match with a or b.
• (.*) Match with any character zero or more times and capture this as group 2
• \1 Match with the captured group, if the group captured "a" the \1 means "a". basically, it means what you match in start using that in this place.

CodePudding user response：

You could use

(([ab]).*?\2)

• ( Capturing group
  • ( Capturing group
    • [ab] Match a or b character
  • ) Close group
  • .*?\2 Non-greedy match till the second group
• ) Close group

See the demo

CodePudding user response：

As you do not want to capture a and b markers themself, you can use a positive lookbehind and lookahead.

As marker should be the same at the end as at the start you can use a capturing group in a lookaround.

So regex would be like this

(?<=([ab]))(.*)(?=\1)

Please, check a demo https://regex101.com/r/ebZRDz/3