c:\@Work\Perl\monks>perl -wMstrict -le
"for my $string (qw(
   50.10-d557_5  50.10-d557_8  50.10-d557_20  50.10-d557_123
   )) {
   my ($value) = $string =~ m{ _ (\d+) \z }xms;
   print qq{'$string' -> '$value'};
   }
"
'50.10-d557_5' -> '5'
'50.10-d557_8' -> '8'
'50.10-d557_20' -> '20'
'50.10-d557_123' -> '123'

[download]
Please see perlre, perlretut, and perlrequick. (Update: Also see articles in the Tutorials -> Pattern Matching, Regular Expressions, and Parsing section.)

Update: For completeness, here's code showing capture and use of matching/parsing success status, and also some edge-case and failing parse examples. Data::Dumper::Dumper(), which is core, can be used instead of non-core Data::Dump::dd(). 

c:\@Work\Perl\monks>perl -wMstrict -e
"use Data::Dump qw(dd);
 ;;
 for my $string (qw(
   50.10-d557_5  50.10-d557_8  50.10-d557_20  50.10-d557_123
   _5  ___6  9_99_999_321
   999_x  999_7x  999_  999  _
   )) {
   my $got_value = my ($value) = $string =~ m{ _ (\d+) \z }xms;
   dd $string, $value, $got_value;
   if ($got_value) {
     print qq{'$string' -> '$value' \n\n};
     }
   else {
     print qq{'$string' parse failed \n\n};
     }
   }
"
("50.10-d557_5", 5, 1)
'50.10-d557_5' -> '5'

("50.10-d557_8", 8, 1)
'50.10-d557_8' -> '8'

("50.10-d557_20", 20, 1)
'50.10-d557_20' -> '20'

("50.10-d557_123", 123, 1)
'50.10-d557_123' -> '123'

("_5", 5, 1)
'_5' -> '5'

("___6", 6, 1)
'___6' -> '6'

("9_99_999_321", 321, 1)
'9_99_999_321' -> '321'

("999_x", undef, 0)
'999_x' parse failed

("999_7x", undef, 0)
'999_7x' parse failed

("999_", undef, 0)
'999_' parse failed

(999, undef, 0)
'999' parse failed

("_", undef, 0)
'_' parse failed

[download]

Give a man a fish:  <%-{-{-{-<

In reply to Re: regex to extract value from a string (updated) by AnomalousMonk
in thread regex to extract value from a string by Anonymous Monk

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