in reply to substring/regex question

Update: I may have misread your question. I missed the one little word in your description that changes the whole premise: "sort". I now believe you actually want to print the input file sorted on the 3rd column, not just print the 3rd column. I'd use the Schwartzian Transform for a pure-Perl solution:

    print for  map { pop @$_ }
              sort { $a->[0] <=> $b->[0] }
               map { [ (split /,/)[3], $_ ] } <>;

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Of course, the following non-Perl solution works just as well on your UNIX-ish OS:

sort -t, -k4 -n <trace.csv >rgextract.txt
#!/opt/local/bin/perl -w

It appears you're on OS X or other UNIX-ish OS, so given this, and the apparent regularity of your data, I'd probably use the following one-liner:

perl -F, -anE 'say $F[3]' <trace.csv >rgextract.txt

See perlrun for information on the switches used, but basically:

    
    -F,     Autosplit delimiter is ','
    -a      Enable autosplit fields into @F
    -n      Assume while (<>) { ... } around program
    -E      Eval one-line program with optional features
            (i.e., 'say', in this case)

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use strict; use warnings; omitted for brevity.

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