Perhaps you should look over
perl regular expressions
Tutorials (check out the few perlre tuts there).
Here is the output from japhy's YAPE::Regex::Explain mod

pattern:   /\.([^\.]*)$/
(?-imsx:      group, but do not capture (case-sensitive)
             (with ^ and $ matching normally) (with . not
             matching \n) (matching whitespace and #
             normally):
\.           '.'
(            group and capture to \1:
[^\.]*       any character except: '\.' (0 or more
             times (matching the most amount
             possible))
)            end of \1
$            before an optional \n, and the end of the
             string
)            end of grouping
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-bn