Read and return a regular expression as a string.
When PROMPT doesn't end with a colon and space, it adds a final ": ".
If DEFAULTS is non-nil, it displays the first default in the prompt.
Non-nil optional arg DEFAULTS is a string or a list of strings that
are prepended to a list of standard default values, which include the
string at point, the last isearch regexp, the last isearch string, and
the last replacement regexp.
Non-nil HISTORY is a symbol to use for the history list.
If HISTORY is nil, `regexp-history' is used.