RegEx - match where second letter is not first letter and third letter is first letter-CodePudding

I'm looking for a regex that will identify expressions where the first and third letters are the same, but different to the second letter.

e.g. match against the following expressions:

abaxyz
bzbaaadsfsdf

but not

aaaxyz
abcdefg
bbbaaasdf

I've tried back-references, but can't make "NOT match 1"

Imagining something like ^(?[a-z]){!P!}{P1}

where P1 is the capture of the first letter, {!P1} is "anything except the first capture" and {P1} is "same as the first capture".

I have more complex requirements to follow, such as "first letter, not first letter, not first or second letter" (i.e. 3 distinct letters at the start)

e.g.

abcdef

but not

abbxyzz

So if anyone can point me to the proper regex constructs for referencing and matching/excluding previous matches/captures, that would much appreciated. Thanks.

CodePudding user response：

You can use

^([a-z])(?!\1)[a-z]\1

See the regex demo. Details:

^ - start of string
([a-z]) - Group 1: a lowercase ASCII letter
(?!\1) - a negative lookahead that fails the match if there is Group 1 value immediately to the right of the current location
[a-z] - a lowercase ASCII letter
\1 - (an inline backreference) Group 1 value.

If you want to match the rest of the word, append the [a-z]*$ to the regex.