Regex for a string with an optional non-repeating dot

Question

I need to validate a string in Java to fulfill the following requirements:

• string must be 5-32 characters long
• string can contain
  • letters (az),
  • numbers (0-9),
  • dashes (-),
  • underscores (_),
  • and periods (.).
• string mustn't contain more than one period (.) in a row.

Would this regex be be a correct solution?

^(?!([^\\.]*+\\.){2,})[\\.a-z0-9_-]{5,32}$

Answer 1

You're pretty close, You can use this regex to block 2 periods in input:

^(?!([^.]*\\.){2})[.a-z0-9_-]{5,32}$

If you want to block 2 consecutive dots then use:

^(?!.*?\\.{2})[.a-z0-9_-]{5,32}$

Answer 2

I love regular expressions, but for reasons of readability and maintainability, I think they should be kept simple wherever possible, and that means using them for what they're good at, and using other features of your language/environment where appropriate.

In the comments you say this is for bean validation. You could validate your field with multiple simple annotations:

@Size(min=5,max=32)
@Pattern.List({
    @Pattern(regexp = "^[a-z0-9-_.]*$", 
             message = "Valid characters are a-z, 0-9, -, _, ."),
    @Pattern(regexp = "^((?!\.{2}).)*$", 
             message = "Must not contain a double period")
})
private String myField;

Also bear in mind that you can write custom constraints in Java.

... and of course in other contexts the same applies:

boolean isValid(String s) {
    return s.length() >= 5 &&
           s.length() <= 32 &&
           s.matches("^[a-z0-9-_.]*$") &&
           !s.contains("..")
}

Answer 3

This might work

 #   "^(?:[0-9a-z_-]|\\.(?!\\.)){5,32}$"  

 ^ 
 (?:
      [0-9a-z_-] 
   |  \.
      (?! \. )
 ){5,32}
 $