Determine if a string is a valid IPv4 address in C

Question

What would be a good way to determine if a string contains an IPv4 address? Should I use isdigit()?

Answer 1

In the url/uri rfc 3986, the Augmented Backus-Naur Form (ABNF) ipv4 address is defined as:

  IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet

  dec-octet   = DIGIT                 ; 0-9
              / %x31-39 DIGIT         ; 10-99
              / "1" 2DIGIT            ; 100-199
              / "2" %x30-34 DIGIT     ; 200-249
              / "25" %x30-35          ; 250-255

I implemented the check with regexp in the following form:

// Although the RFC says ipv6 octects like 001 are not valid, it would be risky
// not to accept those
#define decoct "([01]?[0-9]?[0-9]|2[0-4][0-0]|25[0-5])"
#define ipv4 "(" decoct "\\." decoct "\\." decoct "\\." decoct ")"