"Read and return a regular expression as a string.
Prompt with the string PROMPT. If PROMPT ends in \":\" (followed by
optional whitespace), use it as-is. Otherwise, add \": \" to the end,
-possible preceded by the default result (see below).
+possibly preceded by the default result (see below).
The optional argument DEFAULTS can be either: nil, a string, a list
of strings, or a symbol. We use DEFAULTS to construct the default