The tags API has a "filter" parameter but it only matches the exact substring in the parameter in the tags.
On Travel we have one kind of tags, IATA airport codes (SYD for Sydney, SFO for San Francisco, etc) which would match a pattern of having only three characters. But as each character can be any letter of the alphabet, the current filter doesn't help and we have to fetch all the tags and filter the three-letter ones externally.
Would there be enough other sites with tags matching some kind of pattern for something like this to be useful? As sites get quite large and have lots of tags this query will become more and more resource heavy so a filtered version would use less bandwidth but perhaps more CPU on the server end.