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'.