Dfyjkt

I am implementing a "bank" software and I have to take in an acceptable username, pin, and balance when creating a new user.

BANK: create-user bob 1234 11111

would be an example of a correct input

BANK: create-user bob0101 1234 11111

would be incorrect, basically a single word username, a 4 digit pin, and some integer.

regexec returns 0 for both username inputs even though it absolutely shouldn't.

the code is currently

regex_t regex_s, regex_i, regex_p;
int reti_s = regcomp(&regex_s, "[a-z A-Z]+", REG_EXTENDED);
.
.
.
reti_s = regexec(&regex_s, cmd[1], 0, NULL, 0);

where cmd[1] is the username

I don't have a lot of experience with C regex but I know this shouldn't allow numbers, any help would be great thanks!

If you want to match letters-only characters, you have to use something like this:

int reti_s = regcomp(&regex_s, "[a-z]+", REG_EXTENDED|REG_ICASE);

I added the REG_ICASE flag according to https://linux.die.net/man/3/regcomp.

regexec just returns 0 if the regexp simply matches. If you want to extract information, you need to use an array of regmatch_t to store the matching of subexpressions. Or if you are only interested in global matches, one array of just one regmatch_t element (to get the info about the match).

As the manual page says:

The 0th member of the pmatch array is filled in to indicate what substring of string was matched by the entire RE.

and you have to use the rm_so and rm_eo fields of regmatch_t to get the initial position and the end position of the matched part of the string.