A character called a delimiter usually marks the beginning and end of a regular expression. The delimiter is always a special character for the regular expression it delimits (that is, it does not represent itself but marks the beginning and end of the expression). Although vi permits the use of other characters as a delimiter and grep does not use delimiters at all, the regular expressions in this appendix use a forward slash (/ ) as a delimiter. In some unambiguous cases, the second delimiter is not required. For example, you can sometimes omit the second delimiter when it would be followed immediately by RETURN.