Example of (Crude) Lexical Analysis

$ident = "[a-zA-Z][a-zA-Z0-9]*";

$int = "[\-]?[0-9]+";

$op = "[\-\+\*\/\=]|mod";

$exp = "begin x = 5; print sqrt(x); end";

$exp =~ s/$ident/ID/g;

$exp =~ s/$int/N/g;

$exp =~ s/$op/OP/g;

print $exp;

ID ID OP N; ID ID(ID); ID