triyng to lower case the to letters and replace the digits with yy

It would help if you'd include a sample of the desired output, rather than just a description and sample code. However, if I understand you correctly, this will do:

    $file =~ s/^(\w\w)\d+/\L$1yy/;

Outputs:

xyyy.TXT
twyy.TXT
kgyy.TXT
mmyy.TXT

[download]

If you need the entire filename in lowercase, I might use another statement for clarity:

    $file =~ s/\d+/yy/;
    $file =  lc $file;

[download]
use strict; use warnings; omitted for brevity.

In reply to Re: RegEx match in files by rjt
in thread RegEx match in files by Anonymous Monk

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