in reply to
How will my regular expression match?
Though not directly related to the explanation here, I think this is the most relevant place.
There are 6 Patterm Matching Rules explained for Perl Engine (Programming Perl by Larry, Tom and Randal). But in Rule 3 what is the meaning of the Left to Right Pecking order as I understand that each and every item (Assertion or Atom) of the current RegEx Alternative is searched sequentially in the String from the current position onwards.
So in case the current item does not match at current position in the string, what does it mean to try the next higher pecking order items.