I had some fun with this. My shortest fully-functional attempt is this 60-character regexp:

s/\b(qu|y(?=[^t])|[^\W\daeiouy]*)([a-z']+)/$2.($1||w).ay/eg;

with these features:

• Handles multiple consonants at start of word (and handles 'qu' correctly)
• Correctly handles y-related idiosyncrasies: yummy becomes ummyyay, but yttrium becomes yttriumway and rhythm becomes ythmrhay
• Handles numbers correctly (42 doesn't become 42ay)
• Counts apostrophe as part of word, so "don't" becomes "on'tday" (and handles other punctuation correctly)

Note that if we ignore two words (yttrium and ytterbium), we can safely bring it down to 52 chars:

s/\b(qu|y|[^\W\daeiouy]*)([a-z']+)/$2.($1||w).ay/eg;

And if we decide to switch to the dialect of pig latin that doesn't add 'w' on vowel-words, it's down to 44:

s/\b(qu|y|[^\W\daeiouy]*)([a-z']+)/$2$1ay/g;

If I combine that with perlmonkey's attempt (losing a bit of functionality in the process, although it still works pretty well) I can reach 36:

s/(qu|y|[^aeiouy\s]*)(\w+)/$2$1ay/g;

Also note that my (un-golfed) attempt at a pig latin converter in Visual Basic took up almost 3500 characters (!) without handling nearly all of the exceptions mentioned here. This is (one of the many reasons) why I love Perl!

Thanks to everyone for their previous attempts, which helped me quite a bit. Comments are welcome.

-davisagli