I am trying to validate a regex which

• allows 10 digits
• if digit starts with 672 then it should only allow total 9 digits

I have tried below regex
/^\d{10}$|^(672)\d{6}$/

https://regex101.com/r/0ahnKx/1

It works for 10 digits but if number starts with 672 then also it allows 10 digits.
Could anyone help how can I fix this?
Thanks!

>Solution :

First of all, the capturing group in your regex is redundant, it would make sense to wrap the part of the pattern between ^ and $ to only use single occurrences of the anchors.

To fix the issue, you need to make sure the first three digits matched by \d{10} are not 672, and you can achieve that with a negative lookahead:

/^((?!672)\d{10}|672\d{6})$/
/^(?:(?!672)\d{10}|672\d{6})$/

See the regex demo. Details:

• ^ – start of string
• (?: – start of a group:
  • (?!672)\d{10} – no 672 substring check is triggered and then ten digits are matched
• | – or
  • 672\d{6} – 672 and six digits
• ) – end of the group
• $ – end of string.