The LEX to filter the IPv4 format.

IPv4.l

%{
    #include <iostream>
    #include <string>
%}

integer_in_ipv4    0|1([0-9][0-9]?)?|2([5-9]|[0-4][0-9]?|5[0-5]?)?|[3-9][0-9]?
ipv4               {integer_in_ipv4}\.{integer_in_ipv4}\.{integer_in_ipv4}\.{integer_in_ipv4}

%%

{ipv4} {ECHO; printf("\n");}
.
\n

%%

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