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Obsolete (``basic'') regular expressions differ in several
respects. `|', `+', and `?' are ordinary characters and
there is no equivalent for their functionality. The
delimiters for bounds are `\{' and `\}', with `{' and `}'
by themselves ordinary characters. The parentheses for
nested subexpressions are `\(' and `\)', with `(' and `)'
by themselves ordinary characters. `^' is an ordinary
character except at the beginning of the RE or the begin<AD>
ning of a parenthesized subexpression, `$' is an ordinary
character except at the end of the RE or the end of a
parenthesized subexpression, and `*' is an ordinary char<AD>
acter if it appears at the beginning of the RE or the
beginning of a parenthesized subexpression (after a possi<AD>
ble leading `^'). Finally, there is one new type of atom,
a back reference: `\' followed by a non-zero decimal digit
d matches the same sequence of characters matched by the
dth parenthesized subexpression (numbering subexpressions
by the positions of their opening parentheses, left to
right), so that (e.g.) `\([bc]\)\1' matches `bb' or `cc'
but not `bc'.
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