JavaScript Regex restrict _

Question

This is my regex

^(?![^a-z])(?!.*\.\.)[a-z0-9._]+([a-z])$

Rules

• String should start and end with [a-z]

• double . in a row is not allowed

• double _ in a row is allowed ( should be not this is wrong)

• Allowed characters are [a-z0-9_.]

everything works as like what i want, but i cannot restrict 2 o more _ in a row it has same restrict for . but does not work for _

This is online editor: https://regex101.com/r/XJXlpS/2

what is wrong ?

Answer 1

You can use this regex:

/^(?![^a-z])(?!.*([_.])\1)[\w.]*[a-z]$/gmi

RegEx Demo

RegEx Breakup:

• ^: Start
• (?![^a-z]): Make sure first char is a letter
• (?!.*([_.])\1): Make sure we don't have repeated underscore or dot
• [\w.]*: Match 0+ word characters or dot
• [a-z]: Match a letter in the end
• $: End