Java – Regular expressions used to validate custom formats

I have this format: xx:xx:xx or xx

:xx:xx-y, where x can be 0-9 a-f A-F and y can only be 0 or 1.

I came up with this regular expression: ([0-9A-Fa-f]{2}[:][0-9A-Fa-f]{2}[:][0-9A-Fa -f]{2}| -][0-1]{1})

(See regexr).

But this also matches 0a:0b:0c-3, which is not expected.

Is there a way to remove these cases from the results?

[:] indicates that the list contains only : characters. It is with
:。 The result of [-] is the same as - .
In addition, {1} means “exactly once in the previous paragraph”. It does nothing and you can remove it completely.

To match xx:xx:xx or xx:xx

:xx-y, the part that matches -y must be optional. The quantifier ? after the optional section marks it as optional.

All in all, your regex should look like this:

[0-9A-Fa-f]{2}:[0-9A-Fa-f]{2}:[0-9A-Fa-f]{2}(-[01])?

If you can tell you to use the regex engine to ignore character case, you can remove all character classes from A-F (or a-f) and regex becomes:

[0-9a-f]{2}:[0-9a-f]{2}:[0-9a-f]{2}(-[01])?

How it works, block by block:

[0-9a-f]      # any digit or letter from (and including) 'a' to 'f'
{2}           # the previous piece exactly 2 times
:             # the character ':'
[0-9a-f]
{2}
:
[0-9a-f]
{2}
(             # start a group; it does not match anything
  -           # the character '-'
  [01]        # any character from the class (i.e. '0' or '1')
)             # end of group; the group is needed for the next quantifier
?             # the previous piece (i.e. the group) is optional
              # it can appear zero or one times

See the effect in action: https://regexr.com/4rfvr

Update

As mentioned by @the-fourth-bird in the comment, if regex has to match the entire string, then you need to anchor its end:

^[0-9a-f]{2}:[0-9a-f]{2}:[0-9a-f]{2}(-[01])?$

^ as the beginning of the first character match string of

regex, $ as the end of the last character match string. This way regex only matches the entire string (when there are no other characters before and after xx:xx:xx or xx:xx:xx- before or after the y part).

If you use regex to find xx:xx:xx or xx:xx:xx-y in a larger string, then you don’t need to add ^ and $. Of course, you can just add ^ or $ to have regex match only the beginning or end of the string.