Louise, you're almost there. But the $1 and $2 have a crucially important limitation (actually, a feature): they only get “set” to a Found Text value for Find items that are bracketed with round parentheses. That way you can be very selective about what found text you re-use in the Replace With text. Try this Find What:
(d,)(d)
and replace with your own “$1 space $2″.
The first “found text”, to be stored into $1, will be any digit followed by a comma; the second set, $2, will contain the digit after that comma. When replacing, ID writes back digit #1 and comma, then the extra space you inserted into the Replace field, then digit #2. The parentheses themselves do not influence the search text at all.
I am using “d” above to look for any single digit. So this will find any digit-comma-any digit; it doesn't matter if there are more digits before or after each of these.
Using your parentheses correctly to mark the to-be-copied text is important, because suppose you want to find that comma but replace it with a semi-colon. In that case, you would not include the comma inside, but use “(d),(d)” instead and add the semicolon with “$1; $2″.
You can also do other stuff, such as exchange the order of found items — look for “Second name, First Name” and replace with “First name Second name”, for example. And it's even possible to re-use the “found text” immediately in the Find Text field! This is done with a slightly different syntax, you need to use “1″, “2″ (etc.) instead of “$1″, “$2″. Try this one for fun: search for “(bw+) 1″ — it will magically find double words!
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