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perl/Examples/Chap8/expr-parser.pl

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+
+
+###
+### simple-expr-parser.pl
+###
+
+## Chapter 8 section 4
+
+use Parser ':all';
+use Lexer ':all';
+
+my $expression;
+my $Expression = parser { $expression->(@_) };
+$expression = alternate(concatenate(lookfor('INT'),
+                                    lookfor(['OP', '+']),
+                                    $Expression),
+                        concatenate(lookfor('INT'),
+                                    lookfor(['OP', '*']),
+                                    $Expression),
+                        concatenate(lookfor(['OP', '(']),
+                                    $Expression,
+                                    lookfor(['OP', ')'])),
+                        lookfor('INT'));
+my $entire_input = concatenate($Expression, \&End_of_Input);
+my @input = q[2 * 3 + (4 * 5)];
+my $input = sub { return shift @input };
+my $lexer = iterator_to_stream(
+               make_lexer($input,
+                       ['TERMINATOR', qr/;\n*|\n+/                 ],
+                       ['INT,         qr/\d+/                      ],
+                       ['PRINT',      qr/\bprint\b/                ],
+                       ['IDENTIFIER', qr|[A-Za-z_]\w*|             ],
+                       ['OP',         qr#\*\*|[-=+*/()]#           ],
+                       ['WHITESPACE', qr/\s+/,          sub { "" } ],
+               )
+             ); 
+if (my ($result, $remaining_input) = $entire_input->($lexer)) {
+  use Data::Dumper;
+  print Dumper($result), "\n";
+} else {
+  warn "Parse error.\n";
+}