The question mark (?) in flex

Question

I cannot understand what the question means in a regular expression in flex. For example, in my notes it says:

[ a-z ]? corresponds to 0 or 1 repetition (used for optional parts).

and another example says:

-?[1-9][ 0-9 ]∗ corresponds to any nonzero integer.

I do not understand why these regular expressions correspond to their explanations. Can somebody explain these two?

Answer 1

The ? isn't specific to flex it is a part of most regular expression engines. It basically means that whatever comes before the ? is optional, so it doesn't actually have to be there.

Specifically it means "Zero or one of the preceding element".

Examples:

Nov(ember)?              // matches November or Nov
Feb(ruary)? 23(rd)?      // matches Feb 23 or February 23 or February 23rd or Feb 23rd
colou?r                  // matches color or colour