I want to create a regular expression which should satisfy below requirement.

1. a-z and A-Z allowed.
2. 0-9 allowed.
3. no special chars, no space.
4. no 2 repeating chars allowed.
5. length should be between 6-9 length.

here are some example

abcdef - allowed

abc456 - allowed

Abc685 - allowed

aabc123 - not allowed because 2 aa

sdf112 - not allowed because 2 11

aerwet2345 - not allowed because length > 9

^(?:([a-zA-Z0-9])(?!.\1))$

I have tried this regular expression but seems like repeating 2 chars not working.

CodePudding user response：

Try this:

\b(?:([a-zA-Z0-9])(?!\1)){6,9}\b

Using [a-zA-Z0-9] already makes sure, special characters are not included, so you won't need . in the negative lookahead. Also, using {6,9} on the non capturing group (?:) limits the results to those with 6, 7, 8 or 9 characters.

If your 'word' starts at the beginning of each line and nothing follows your 'words' on each line, you can/should use ^ and $ instead of the word boundaries \b, as you did in your example.

CodePudding user response：

Your pattern ^(?:([a-zA-Z0-9])(?!.\1))$ only matches a single character with ([a-zA-Z0-9]) so the assertion as no purpose.

The assertion (?!.\1) checks that the single captured value is not directly followed by any char except a newline and a backreference to group 1, and as the .\1 matches a single char before the backreference it does not guarantee 2 of the same repeating chars

You could assert not 2 of the same chars from the character class, and then match 6-9 chars.

^(?![a-zA-Z0-9]*([a-zA-Z0-9])\1)[a-zA-Z0-9]{6,9}$

Regex demo