Please, format you messages according to Markup in the Monastery to made them readable!

I need to pattern match the root filename, and display the latest version of file:

12345_Av1.pdf
123456_Bv1.pdf
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Your examples don't define well what is `root filename' and `version'. Assuming `the root' is anything before _ and `version' is everything after _.v, the following expressions will match them in $1 and $2, respectively:

if ($filename =~ /^([^_]+)_.*v(\d+)/) {
    # do something with $1 and $2 here
}
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You can find more about regular expressions in e.g. perlretut.

Update: typos corrected