When using regex in C, \d does not work but [0-9] does

Question

I do not understand why the regex pattern containing the \\d character class does not work but [0-9] does. Character classes, such as \\s

Answer 1

Trying either pattern in a strictly POSIX environment will likely end up having no matches; if you want to make the pattern truly POSIX compatible use all bracket expressions:

const char *rstr = "^[[:digit:]]+[[:space:]]+[[:alpha:]]+[[:space:]]+[[:digit:]]+[[:space:]]+[[:alpha:]]+$";

↳ POSIX Character_classes