Java Regex to match String password

Question

I have recently encountered this question in the text book:

I am suppose to write a method to check if a string have:

1. at least ten characters
2. only letters and digits
3. at least three digits

I am trying to solve it by Regx, rather than iterating through every character; this is what I got so far:

String regx = "[a-z0-9]{10,}";

But this only matches the first two conditions. How should I go about the 3rd condition?

Answer 1

You could use a positive lookahead for 3rd condition, like this:

^(?=(?:.*\d){3,})[a-z0-9]{10,}$

• ^ indicates start of string.
• (?= ... ) is the positive lookahead, which will search the whole string to match whatever is between (?= and ).
• (?:.*\d){3,} matches at least 3 digits anywhere in the string.
  • .*\d matches a digit preceded by any (or none) character (if omitted then only consecutive digits would match).
  • {3,} matches three or more of .*\d.
  • (?: ... ) is a non-capturing group.
• $ indicates end of string.