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Running the Scanner
Here is the output of the scanner when executed and given the file main.cpp as input,
i.e., the scanner is being asked to provide tokens found in the file main.cpp:
lex <main.cpp
259,void 258,main 283,( 284,) 285,{ 258,FlexLexer 258,lex 290,; 260,int 258,tc 266,= 258,lex 291,. 258,yylex 283,( 284,) 290,; 263,while 283,( 258,tc 276,!= 257, 284,) 258,cout 279,<< 258,tc 279,<< 292,"," 279,<< 258,lex 291,. 258,YYText 283,( 284,) 279,<< 258,endl 290,; 258,tc 266,= 258,lex 291,. 258,yylex 283,( 284,) 290,; 286,}
Flex input for C++
As an illustration of the power of Flex, here is the input for generating scanner for C++
compiler.
- ISO C++ lexical analyzer.
- Based on the ISO C++ draft standard of December '96. */
%{ #include <ctype.h> #include <stdio.h> #include “tokdefs.h"
int lineno;
static int yywrap(void); static void skip_until_eol(void); static void skip_comment(void); static int check_identifier(const char *); %}
intsuffix ([uU][lL]?)|([lL][uU]?) fracconst ([0-9].[0-9]+)|([0-9]+.) exppart [eE][-+]?[0-9]+ floatsuffix [fFlL] chartext ([^'])|(\.) stringtext ([^"])|(\.) %% %% "\n" { ++lineno; } [\t\f\v\r ]+ { / Ignore whitespace. */ }
"/*" { skip_comment(); } "//" { skip_until_eol(); }
"{" { return '{'; } "<%" { return '{'; } "}" { return '}'; } "%>" { return '}'; } "[" { return '['; } "<:" { return '['; } "]" { return ']'; } ":>" { return ']'; } "(" { return '('; } ")" { return ')'; } ";" { return ';'; } ":" { return ':'; } "..." { return ELLIPSIS; } "?" { return '?'; } "::" { return COLONCOLON; } "." { return '.'; } "." { return DOTSTAR; } "+" { return '+'; } "-" { return '-'; } "" { return '*'; } "/" { return '/'; } "%" { return '%'; } "^" { return '^'; } "xor" { return '^'; } "&" { return '&'; }
"friend" { return FRIEND; } "goto" { return GOTO; } "if" { return IF; } "inline" { return INLINE; } "int" { return INT; } "long" { return LONG; } "mutable" { return MUTABLE; } "namespace" { return NAMESPACE; } "new" { return NEW; } "operator" { return OPERATOR; } "private" { return PRIVATE; } "protected" { return PROTECTED; } "public" { return PUBLIC; } "register" { return REGISTER; } "reinterpret_cast" { return REINTERPRET_CAST; } "return" { return RETURN; } "short" { return SHORT; } "signed" { return SIGNED; } "sizeof" { return SIZEOF; } "static" { return STATIC; } "static_cast" { return STATIC_CAST; } "struct" { return STRUCT; } "switch" { return SWITCH; } "template" { return TEMPLATE; } "this" { return THIS; } "throw" { return THROW; } "true" { return TRUE; } "try" { return TRY; } "typedef" { return TYPEDEF; } "typeid" { return TYPEID; } "typename" { return TYPENAME; } "union" { return UNION; } "unsigned" { return UNSIGNED; } "using" { return USING; } "virtual" { return VIRTUAL; } "void" { return VOID; } "volatile" { return VOLATILE; } "wchar_t" { return WCHAR_T; } "while" { return WHILE; }
[a-zA-Z_][a-zA-Z_0-9]* { return check_identifier(yytext); }
"0"[xX][0-9a-fA-F]+{intsuffix}? { return INTEGER; } "0"[0-7]+{intsuffix}? { return INTEGER; } [0-9]+{intsuffix}? { return INTEGER; }
{fracconst}{exppart}?{floatsuffix}? { return FLOATING; } [0-9]+{exppart}{floatsuffix}? { return FLOATING; }
"'"{chartext}"'" { return CHARACTER; } "L'"{chartext}"'" { return CHARACTER; }
"""{stringtext}""" { return STRING; } "L""{stringtext}""" { return STRING; }
. { fprintf(stderr, "%d: unexpected character `%c'\n", lineno, yytext[0]); }
static int yywrap(void)
return 1; } static void skip_comment(void) { int c1, c2;
c1 = input(); c2 = input();
while(c2 != EOF && !(c1 == '*' && c2 == '/')) { if (c1 == '\n') ++lineno; c1 = c2; c2 = input(); } } static void skip_until_eol(void) { int c;
while ((c = input()) != EOF && c != '\n') ; ++lineno; }
static int check_identifier(const char s) { /
- This function should check if `s' is a
- typedef name or a class
- name, or a enum name, ... etc. or
- an identifier. */ switch (s[0]) { case 'D': return TYPEDEF_NAME; case 'N': return NAMESPACE_NAME; case 'C': return CLASS_NAME; case 'E': return ENUM_NAME; case 'T': return TEMPLATE_NAME; } return IDENTIFIER; }
Parsing
We now move the second module of the front-end: the parser. Recall the front-end
components:
scanner parser
source
code
tokens IR
errors
