How to find comma without following space in MS Word 2003?

You can use this pattern:

[!<]?,[!>]?

That means: find a comma character between any characters, just not at the beginning of the word and not on the end of the word.

So that in the example below, one would find only the comma at "3,a" (but NOT those at: ",1" and "c,")

,123,abc,

I can tell you a simpler way.
Just replace all , with ,<space> where <space> means a blank space and then replace ,<space><space> with ,<space> to account for places where users did put a space after comma.

WARNING: If you use commas in numbers (either as thousand separator or decimal separator), you need to go through the document to correct numbers such as "1,200" which will now be rendered as "1, 200" .