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'.