I recently read an RFC document and I noticed that regex operators that have been used don't match the commonly known. For example:
date-time = [ day-of-week "," ] date time [CFWS]
year = (FWS 4*DIGIT FWS) / obs-year
The square bracket means that it will match only one out of several characters in it. But in the RFC I see that they interpret it as "optionally". The same with the asterix, that says the preceding token will occur zero times or more. In the example we have
4*DIGIT
which is not difficult to guess that means 4 occurences of DIGIT token.
How should I interpret the RFC document regex operators, is there any document describing their designation?
The document (I believe) you're looking at, RFC 2822, says this:
1.2.2. Syntactic notation
This standard uses the Augmented Backus-Naur Form (ABNF) notation specified in [RFC2234] for the formal definitions of the syntax of messages.
So, yes, the syntax is defined in RFC 2234, and is not Regular Expressions.
A few sections specific to the block you've quoted:
3.5 Sequence Group
Elements enclosed in parentheses are treated as a single element, whose contents are STRICTLY ORDERED.
3.6 Variable Repetition
The operator "*" preceding an element indicates repetition. The full form is:
<a>*<b>elementwhere <a> and <b> are optional decimal values, indicating at least <a> and at most <b> occurrences of element.
3.8 Optional Sequence
Square brackets enclose an optional element sequence: