The validator is currently only set up for throws up to f. Any patterns containing throws higher than f will be treated as invalid.
Also note that only lowercase letters for throws over 9 are accepted—Siteswap looks far cleaner that way.
Interesting fact: the regular expression used for validating siteswaps containing throw heights of up to f is 49,458 characters long.
If you want you can
Just remember that the higher the maximum throw height, the longer any given siteswap will take to validate.
After a certain point browsers will stop processing the regular expression due to its length, which results in any input whatsoever being reported as constantly either valid or invalid. Webkit browsers (Chrome and Safari) seem to work up to t, as it is the last maximum throw height for which the regular expression is fewer than 1-million characters long, after which any input is invalidated. Firefox works for u as well, albeit more slowly, after which any input is validated. Opera and Internet Explorer have not been tested.