Looking at the patterns included in the other answers there are a number of good patterns that can be improved by referencing groups and utilizing lookaheads. Here is an example of a pattern that is self referencing that I would utilize in PHP if I had to:
^(?(?[[:xdigit:]]{0,4}) # grab a sequence of up to 4 hex digits
# and name this pattern for usage later
(?2[0-5]|(?:2[0-4]|1{0,1}[0-9]){0,1}[0-9])\.){3}(?&dgroup)
# match our hex group one last time
|(?&hex))$
Note: PHP has a built in filter for this which would be a better solution than this
pattern.
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