This Tutorial may also be helpful to your understanding: Pack/Unpack Tutorial (aka How the System Stores Data).

These are my most frequently used resources for constructing/understanding regexes:

Mastering Regular Expressions
perlrequick
perlretut
perlre
YAPE::Regex::Explain
GraphViz::Regex
The Regex Coach (Win32 only, IIRC)

HTH,