I'm trying to craft a regular expression to locate some parts of a string and manipulate it with expressions. Do we have any REGEX experts in here?

text.sourceText.match(/(?<=\d)ST|ND|RD|TH/g);

I'm trying to locate one of those strings (ST|ND|RD|TH) but only if it follows a number. For some reason the thing returns the string if a word begins with one of those strings. I can't figure out why it's doing that. I'm telling it only look after digits.