One can elaborate on this. Surely, the  % and  d parts are pointless to extract: if anything is extracted, you know it's a format and you know it's a  d format. The leading  0 on the  04 tells you zero versus space lead-padding in an integer format specifier, but padding with spaces in a file name seems a bit dodgy, so you're really only concerned with whether the width is true/false "fixed and leading-zero padded". (Update: Well, if it's never space-padded, I guess the  0 is superfluous and you don't have to worry about it at all: if a width is given, it's fixed!)

c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = 'Img%04d.png';
 ;;
 my $d_format =
 my ($prefix, $fixed, $width, $ext) =
 $string =~ m{ \A (\w+) % (0?) (\d+) d [.] (\w+) \z }xms;
 ;;
 die 'no format' unless $d_format;
 $fixed = length $fixed ? 1 : '';
 ;;
 print qq{string: '$string'};
 print qq{prefix: '$prefix'};
 print qq{ fixed: '$fixed'};
 print qq{ width: '$width'};
 print qq{   ext: '$ext'};
"
string: 'Img%04d.png'
prefix: 'Img'
 fixed: '1'
 width: '4'
   ext: 'png'
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One can imagine extending this approach to the  %s format with various min/max widths, justification, etc.