In summary, regexps can be:
abcd -- matching a string literally
. -- matching everything except NULL
[a-z_?], ^[a-z_?], [[:alpha:]] and
[^[:alpha:]] -- matching character sets
\(subexp\) -- grouping an expression into a subexpression.
\n -- match a copy of whatever was matched by the nth subexpression.
The following special characters and sequences can be applied to a character, character set, subexpression, or backreference:
* -- repeat the preceeding element 0 or more times.
\+ -- repeat the preceeding element 1 or more times.
\? -- match the preceeding element 0 or 1 time.
{m,n} -- match the preceeding element at least m, and as
many as n times.
regexp-1\|regexp-2\|.. -- match any regexp-n.
A special character, like . or * can be made into a literal
character by prefixing it with \.
A special sequence, like \+ or \? can be made into a
literal character by dropping the \.
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