Lex rule for generating lexical analyzer to match US phone numbers

flex.md

Lex rule for generating lexical analyzer to match US phone numbers.

%{
    int isValid = 0;
%}

digit   [0-9]
valid   {digit}{3,3}-{digit}{3,3}-{digit}{4,4}

%%
{valid}  {isValid = 1;}
. {isValid = 0;}
\n  {return 0;}
%%

int yywrap() {}
int main() {
    while(1) {
        yylex();
        if(isValid) {
            printf("Valid!\n");
        }
        else {
            printf("Invalid!\n");
        }
    }

    return 0;
}

To compile & Run

$ sudo apt update
$ sudo apt install flex
$ lex phnum.l
$ gcc lex.yy.c
$ ./a.out