RegEx No more than 2 identical consecutive characters and a-Z and 0-9

by

http://regexr.com?34vo9

The regular expression:

^(?=.{8,20}$)(([a-z0-9])\2?(?!\2))+$

The first lookahead ((?=.{8,20}$)) checks the length of your string. The second portion does your double character and validity checking by:

(
  ([a-z0-9])      Matching a character and storing it in a back reference.
  \2?             Optionally match one more EXACT COPY of that character.
  (?!\2)          Make sure the upcoming character is NOT the same character.
)+                Do this ad nauseum.
$                 End of string.

Okay. I see you’ve added some additional requirements. My basic forumla still works, but we have to give you more of a step by step approach. SO:

^...$

Your whole regular expression will be dropped into start and end characters, for obvious reasons.

(?=.{n,m}$)

Length checking. Put this at the beginning of your regular expression with n as your minimum length and m as your maximum length.

(?=(?:[^REQ]*[REQ]){n,m})

Required characters. Place this at the beginning of your regular expression with REQ as your required character to require N to M of your character. YOu may drop the (?: ..){n,m} to require just one of that character.

(?:([VALID])\1?(?!\1))+

The rest of your expression. Replace VALID with your valid Characters. So, your Password Regex is:

^(?=.{8,20}$)(?=[^A-Za-z]*[A-Za-z])(?=[^0-9]*[0-9])(?:([\w\d*?!:;])\1?(?!\1))+$

‘Splained:

^
  (?=.{8,20}$)                 8 to 20 characters
  (?=[^A-Za-z]*[A-Za-z])       At least one Alpha
  (?=[^0-9]*[0-9])             At least one Numeric
  (?:([\w\d*?!:;])\1?(?!\1))+  Valid Characters, not repeated thrice.
$

http://regexr.com?34vol Here’s the new one in action.

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