\item[\code{\e( \e)}] Indicates the start and end of a group; the

contents of a group can be matched later in the string with the

\code{\e [1-9]} special sequence, described next.

Matches the contents of the group of the same

number. For example, \code{\e (.+\e ) \e \e 1} matches 'the the' or

'55 55', but not 'the end' (note the space after the group). This

special sequence can only be used to match one of the first 9 groups;

groups with higher numbers can be matched using the \code{\e v}

sequence. (\code{\e 8} and \code{\e 9} don't need a double backslash

because they are not octal digits.)

\item[\code{\e \e b}] Matches the empty string, but only at the

beginning or end of a word. A word is defined as a sequence of

alphanumeric characters, so the end of a word is indicated by