This example demonstrates how to use surrounding
  text to identify a string to be replaced without also
  replacing the surrounding text.

  The following filter replaces the month part of
  a date with the string "mmm". For example, the
  string "14-OCT-1999" will be replaced by the string
  "14-mmm-1999":

     day := any(" 123") + digit;
     month := "JAN" | "FEB" | "MAR" | "APR" |
              "MAY" | "JUN" | "JUL" | "AUG" |
              "SEP" | "OCT" | "NOV" | "DEC";
     year := any(digits,4);
     date := (day + "-"@day_part) + month + ("-" + year@year_part);
     global_replace( date, 'str(day_part) + "mmm" + str(year_part)',,ON);

  The day part of the date and the "-" character are
  assigned to the partial pattern variable day_part and
  the year part of the date and preceding "-" assigned
  to year_part. These partial pattern variables are then
  included in the replacement string.

  When partial pattern variable are used in the
  replacement string they must be evaluated for
  each replacement. To do this, set the parameter
  evaluate_replacement to ON, as shown above.

  When the replacement string is to be evaluated, string
  literals must be nested inside further quotes. This
  is most easily done by using single quotes for the
  outer string and double quotes for any nested string
  literals, or vice-versa. Also, any partial pattern
  variables must be converted to strings using the DECTPU
  procedure STR.

  Note that including LINE_END in the definition of a
  partial pattern variable does not have the effect
  of retaining the line break. See example 6 for a
  resolution of this problem.