Password Regex

[Scott McGhee]

I'm struggling to figure out a single regular expression which will match a password having the following rules:

o Passwords must be 6-16 characters.
o Allowed characters are alphanumeric, hyphen(-) and underscore(_)
o They must contain both numbers and letters

Currently the code uses 3 regular expressions to accomplish the goal:

String regex1 = "^[a-zA-Z0-9_-]{6,24}";
String regex2 = "^.*[a-zA-Z].*$";
String regex3 = "^.*[0-9].*$";

And then to check the password:

if( password.matches(regex1)&&password.matches(regex2)&&password.matches(regex3)){}

What is the easiest way to construct a single regular expression combining these 3?

[Jeanne Boyarsky]

Scott,
The easiest thing to do is leave them separate. I can't think of any way to check all three of these things. Note your requirements say the max length is 16 and your reg exp is 24. I assume one is a typo, but I figured I would mention it.

[Mike Simmons]

You can use lookahead or lookbehind operators to look ahead without advancing. Lookahead is probably a little simpler:

String regex = "^(?=.*[a-zA-Z])(?=.*[0-9])[a-zA-Z0-9_-]{6,24}$";

[Scott McGhee]

Thank you Mike, that clears it up for me. The lookahead seems to be working, I tested on a few input strings and it looks good.

Also thanks Jeanne for pointing out the typo.

[Jeanne Boyarsky]

Mike,
Interesting! I was puzzled by doing the look ahead for two things. Double lookahead is a good technique to remember!