The Lookahead Operator[slide by John Werth.]
If r1 and r2 are patterns, then r1/r2 means match r1 only if it is followed by r2.
For example, Fortran has a DO statement, similar to the for statement; unfortunately, the DO statement could look very similar to an assignment statement:
DO 10 I = 1,25 cf. for (i=1; i<=25; i++) DO 10 I = 1.25 do10i = 1.25;
DO/({letter}|{digit})*=({letter}|{digit})*,
recognizes the keyword DO in the string DO5I=1,25