match one of the first 99 groups. If the first digit of \var{number}
is 0, or \var{number} is 3 octal digits long, it will not be interpreted
as a group match, but as the character with octal value \var{number}.
-(There is a group 0, which is the entire matched pattern, but it can't
-be referenced with \regexp{\e 0}; instead, use \regexp{\e g<0>}.)
Inside the \character{[} and \character{]} of a character class, all numeric
escapes are treated as characters.