Regex to not allow consecutive digits at start and end but to allow middle of string

Question

The regex ^(?!.*(d)1{5})[6-9]d{9}$ will not allow more than 5 consecutive same digits. but how to change regex to allow 6 consecutive digits in middle but not at start and end of string.

The regex should accept 6 consecutive same digits in middle of string like from 9888888654 and should not accept at the start of string like 9999994567 and at end of string like 9873555555

Please help.

Answer 1

You could use a negative lookahead asserting that the string does not start or end with 6 consecutive digits.

^(?!(?:(d)1{5}|.*?(d)2{5}$))[6-9]d{9}$

In parts

• ^ Start of the string
• (?! Negative lookahead
  • (?: Non capture group
    • (d) Capture group 1, match 1 digit
    • 1{5} Repeat 5 times the value captured in group 1
    • | or
    • .*?(d) Match 0+ times as least a possible and capture a digit in group 2
    • 2{5}$ , repeat 5 times the value captured in group 2 till the end of the string
  • ) Close non capture group
• ) Close lookahead
• [6-9] Match a digit 6-9
• d{9} Match 9 digit 0-9
• $ End of string

Regex demo