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RegEx

• . any character except newline
• \n newline
• * zero or more copies of the preceding expression
• • one or more copies of the preceding expression
• ? zero or one copy of the preceding expression
• ^ beginning of line
• $ end of line
• a|b a or b
• (ab)+ one or more copies of ab (grouping)
• "a+b" literal "a+b" (C escapes still work)
• [] character class

Example

• abc abc
• abc* ab abc abcc abccc ...
• abc+ abc abcc abccc ...
• a(bc)+ abc abcbc abcbcbc ...
• a(bc)? a abc
• [abc] one of: a, b, c
• [a-z] any letter, a-z
• [a\-z] one of: a, -, z
• [-az] one of: -, a, z
• [A-Za-z0-9]+ one or more alphanumeric characters
• [ \t\n]+ whitespace
• [^ab] anything except: a, b
• [a^b] one of: a, ^, b
• [a|b] one of: a, |, b
• a|b one of: a, b

Struct

• Input is copied to output one character at a time. The first %% is always required, as there must always be a rules section.

... definitions ...
%%
... rules ...
%%
... subroutines ...

• In this example there are two patterns, “.” and “\n”, with an ECHO action associated for each pattern.
• Several macros and variables are predefined by lex. ECHO is a macro that writes code matched by the pattern.
• This is the default action for any unmatched strings.
• Function yywrap is called by lex when input is exhausted. Return 1 if you are done or 0 if more processing is required.
• yylex that is the main entry-point for lex.

%%
. ECHO;
\n ECHO;
%%

int yywrap(void) {
    return 1;
}

int main(void) {
    yylex();
    return 0;
}

• The following example prepends line numbers to each line in a file. Some implementations of lex predefine and calculate yylineno. The input file for lex is yyin and defaults to stdin.

%{
int yylineno;
%}

%%

^(.*)\n printf("%4d\t%s", ++yylineno, yytext);

%%

int yywrap(void) {
    return 1;
}

int main(int argc, char *argv[]) {
    yyin = fopen(argv[1], "r");
    yylex();
    fclose(yyin);
}

• Here is a scanner that counts the number of characters, words, and lines in a file:

%{
int nchar, nword, nline;
%}

%%

\n { nline++; nchar++; }
[^ \t\n]+ { nword++, nchar += yyleng; }
. { nchar++; }

%%

int yywrap(void) {
    return 1;
}

int main(void) {
    yylex();
    printf("%d\t%d\t%d\n", nchar, nword, nline);
    return 0;
}