RegEx: Non-repeating patterns?

Question

I'm wrestling with how to write a specific regex, and thought I'd come here for a little guidance.

What I'm looking for is an expression that does the following:

• Character length of 7 or more
• Any single character is one of four patterns (uppercase letters, lowercase letters, numbers and a specific set of special characters. Let's say #$%@).

(Now, here's where I'm having problems):

• Another single character would also match with one of the patterns described above EXCEPT for the pattern that was already matched. So, if the first pattern matched is an uppercase letter, the second character match should be a lowercase letter, number or special character from the pattern.

To give you an example, the string AAAAAA# would match, as would the string AAAAAAa. However, the string AAAAAAA, nor would the string AAAAAA& (as the ampersand was not part of the special character pattern).

Any ideas? Thanks!

Answer 1

If you only need two different kinds of characters, you can use the possessive quantifier feature (available in Objective C):

^(?:[a-z]++|[A-Z]++|[0-9]++|[#$%@]++)[a-zA-Z0-9#$%@]+$

or more concise with an atomic group:

^(?>[a-z]+|[A-Z]+|[0-9]+|[#$%@]+)[a-zA-Z0-9#$%@]+$

Since each branch of the alternation is a character class with a possessive quantifier, you can be sure that the first character matched by [a-zA-Z0-9#$%@]+ is from a different class.

About the string size, check it first separately with the appropriate function, if the size is too small, you will avoid the cost of a regex check.

Answer 2

First you need to do a negative lookahead to make sure the entire string doesn't consist of characters from a single group:

(?!(?:[a-z]*|[A-Z]*|[0-9]*|[#$%@]*)$)

Then check that it does contain at least 7 characters from the list of legal characters (and nothing else):

^[a-zA-Z0-9#$%@]{7,}$

Combining them (thanks to Shlomo for pointing that out):

^(?!(?:[a-z]*|[A-Z]*|[0-9]*|[#$%@]*)$)[a-zA-Z0-9#$%@]{7,}$