Regular Expression :: Allow 1,2 Or 3 Digits Before Decimal Point

I wrote a regex to edit a decimal number. you're allowed 1,2, or 3
digits before the decimal point, and 0,1,2,or 3digits after the decimal
point. Of course, you can't enter a decimal point without at least a
digit after ("5." is invalid). So here is my regex

pattern=/^d{1,3}(.(?=d))d{0,3}$/

This works fine for every case except an integer. In other words, it
tests false for entering 5, or 567.

I don't see why it tests false for integers. I'm allowing 1-3 digits
before the decimal point, then a decimal point only if the next
character is a digit (the lookahead clause), and then 0-3 digits after
the decimal point.

I've gotten around this problem with other javascript code around
the regex, but I'd just like to know why this "clean" solution doesn't
work.