Phone number Validation in C

Question

I'm attempting to create a program that validates a phone number entered by a user and stores it in a string. The user entering the phone number will be able to enter it as one of the following formats:

(xxx)xxx-xxxx

xxxxxxxxxx

xxx-xxx-xxxx

I can think of several but very tedious ways to validate the phone number ensuring that it follows the specific format. What could be the simplest option?

"I can think of several but very tedious ways" - share them. Also, this likely belongs to Code Review. — Daniel Kamil Kozar, Mar 21 '17 at 22:54
@DanielKamilKozar: Code review would have a serious talk to you if you moved this question. They expect **working** code. I don't see any in the question. — too honest for this site, Mar 21 '17 at 23:00
@Olaf : I do realise that, which is why I asked the OP to share the code in question first. :) — Daniel Kamil Kozar, Mar 21 '17 at 23:02
@DanielKamilKozar: Questions like this one will almost certainly not be ready for CR. — too honest for this site, Mar 21 '17 at 23:03
check this [library from Google](https://github.com/googlei18n/libphonenumber). — Andrey, Mar 21 '17 at 23:06
@Andrey Looks nice, but the Readme there mentions only C++, not C. — Peter - Reinstate Monica, Mar 21 '17 at 23:26
I must assume that this is some kind of assignment; any real-world approach would use either a general regular expression library or something like Andrey's dedicated tool. Note that you [can write a C wrapper around a C++ library](http://stackoverflow.com/questions/199418/using-c-library-in-c-code) if you want to. — Peter - Reinstate Monica, Mar 21 '17 at 23:28
simple solution.. go through the string, add only the number characters to another string, and validate only your new string against a single format instead of worrying about checking for all 3 formats you listed. I assume you won't be dealing with international phone numbers or numbers with letters in them. — yano, Mar 21 '17 at 23:33
Is `sscanf()` OK to use like `int n = 0; sscanf(buf, "(%*1u%*1u%*1u)%*1u%*1u%*1u-%*1u%*1u%*1u%*1u%n", &n); if (n && buf[n]==0) PassFirstPattern(buf);` ? — chux - Reinstate Monica, Mar 22 '17 at 02:55

Petr Skocik · Accepted Answer · 2017-03-21T23:17:06.987

With re2c, you can specify a regex and have it turned into C.

bool verify(char const *YYCURSOR)
{
    char const*YYMARKER;
    /*!re2c
        re2c:define:YYCTYPE = char;
        re2c:yyfill:enable = 0;
        D = [0-9];  
        '('D{3}')'D{3}'-'D{4} | D{10} | D{3}'-'D{3}'-'D{4} '\x00'      { return 1; }
        * { return 0; } 
    */
}
int main(int argc, char **argv) //test it on main arguments
{
    for(int i=1;i<argc;i++)
        printf("%s\t%d\n", argv[i], verify(argv[i]));
}

Re2c will compile it into a well-oiled finite state machine (by oil, I mean gotos, many gotos):

/* Generated by re2c 0.16 on Wed Mar 22 00:08:21 2017 */
#line 1 "verify.c.re"
bool verify(char const *YYCURSOR)
{
    char const*YYMARKER;

#line 8 "verify.c"
{
    char yych;
    yych = *YYCURSOR;
    switch (yych) {
    case '(':   goto yy4;
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy5;
    default:    goto yy2;
    }
yy2:
    ++YYCURSOR;
yy3:
#line 10 "verify.c.re"
    { return 0; }
#line 31 "verify.c"
yy4:
    yych = *(YYMARKER = ++YYCURSOR);
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy6;
    default:    goto yy3;
    }
yy5:
    yych = *(YYMARKER = ++YYCURSOR);
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy8;
    default:    goto yy3;
    }
yy6:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy9;
    default:    goto yy7;
    }
yy7:
    YYCURSOR = YYMARKER;
    goto yy3;
yy8:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy10;
    default:    goto yy7;
    }
yy9:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy11;
    default:    goto yy7;
    }
yy10:
    yych = *++YYCURSOR;
    switch (yych) {
    case '-':   goto yy12;
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy13;
    default:    goto yy7;
    }
yy11:
    yych = *++YYCURSOR;
    switch (yych) {
    case ')':   goto yy14;
    default:    goto yy7;
    }
yy12:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy15;
    default:    goto yy7;
    }
yy13:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy16;
    default:    goto yy7;
    }
yy14:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy17;
    default:    goto yy7;
    }
yy15:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy18;
    default:    goto yy7;
    }
yy16:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy19;
    default:    goto yy7;
    }
yy17:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy20;
    default:    goto yy7;
    }
yy18:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy21;
    default:    goto yy7;
    }
yy19:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy22;
    default:    goto yy7;
    }
yy20:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy23;
    default:    goto yy7;
    }
yy21:
    yych = *++YYCURSOR;
    switch (yych) {
    case '-':   goto yy24;
    default:    goto yy7;
    }
yy22:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy25;
    default:    goto yy7;
    }
yy23:
    yych = *++YYCURSOR;
    switch (yych) {
    case '-':   goto yy19;
    default:    goto yy7;
    }
yy24:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy26;
    default:    goto yy7;
    }
yy25:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy27;
    default:    goto yy7;
    }
yy26:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy28;
    default:    goto yy7;
    }
yy27:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy29;
    default:    goto yy7;
    }
yy28:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy31;
    default:    goto yy7;
    }
yy29:
    ++YYCURSOR;
#line 9 "verify.c.re"
    { return 1; }
#line 373 "verify.c"
yy31:
    yych = *++YYCURSOR;
    switch (yych) {
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':   goto yy32;
    default:    goto yy7;
    }
yy32:
    ++YYCURSOR;
    if ((yych = *YYCURSOR) <= 0x00) goto yy29;
    goto yy7;
}
#line 12 "verify.c.re"

This approach gets you an extra bonus if you get payed by lines of code ;-). — Peter - Reinstate Monica, Mar 21 '17 at 23:30

Phone number Validation in C

1 Answers1