regex - match pattern of alternating characters

Question

I want to match patterns of alternating lowercase characters.

ababababa -> match

I tried this

([a-z][a-z])+[a-z]

but this would be a match too

ababxyaba

Answer 1

You can use this regex with 2 back-reference to match alternating lowercase letters:

^([a-z])(?!\1)([a-z])(?:\1\2)*\1?$

RegEx Demo

RegEx Breakup:

• ^: Start
• ([a-z]): Match first letter in capturing group #1
• (?!\1): Lookahead to make sure we don't match same letter again
• ([a-z]): Match second letter in capturing group #3
• (?:\1\2)*: Match zero or more pairs of first and second letter
• \1?: Match optional first letter before end
• $: End