How to Regex a string

Question

Is it possible to check with regex:

1. the complete string are numbers AND
2. the first character is a 7 or 8 then the complete length of the st

Answer 1

You can use

^(?:[78]\d{10}|1\d{9}|0\d{6}(?:[87]\d|\d1)\d{9})$

See the regex demo

Details

• ^ - start of string
• (?:[78]\d{10}|1\d{9}|0\d{6}(?:[87]\d|\d1)\d{9}) - one of the three alternatives:
  • [78]\d{10} - 7 or 8 and then 10 digits (11 all in all)
  • | - or
  • 1\d{9} - 1 and then 9 digits (10 all in all)
  • | - or
  • 0\d{6}(?:[87]\d|\d1)\d{9} - 0, then 6 digits, then the 8th digit equal to 8 or 7 and any one digit, or any one digit and the 9th digit equal to 1, and then 9 more digits (=18 digits)
• $ - end of string.

Answer 2

Here's the regex: /^[78]\d{10}$/

"^" indicates the start of the line "$" indicates the end of the line \d means digits {10}means exactly 10 [] is a group of valid values

The second question is dependant on wether your Engine supports Lookahead and Lookbehind https://www.regular-expressions.info/conditional.html