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

According to the POSIX regular expression spec:

An ordinary character is any character in the supported character set, except for the ERE special characters listed in ERE Special Characters. The interpretation of an ordinary character preceded by a backslash ( '\' ) is undefined.

So the only characters that can legally follow a \ are:

\^    \.    \[    \$    \(    \)    \|
\*    \+    \?    \{    \\

all of which match the escaped character literally. Trying to use any of of the other PCRE extensions may not work.