In addition to reading the documentation that Corion mentioned, here is one tool that can help decipher regexes: YAPE::Regex::Explain. For example:

use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new('\*')->explain();

The regular expression:

(?-imsx:\*)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \*                       '*'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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