re = /^0+?(\d{2})/m
str = '001 - Filename 001.mp3
009 - Filename 009.mp3
010 - Filename 010.mp3
060 - Filename 060.mp3
086 - Filename 086.mp3
100 - Filename 100.mp3
1000 - Filename 1000.mp3
01001 - Filename 1001.mp3
01 - Filename with two leading numbers.mp3
1 - Filename with one leading number.mp3'
subst = '$1'
result = str.gsub(re, subst)
# Print the result of the substitution
puts result
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Ruby, please visit: http://ruby-doc.org/core-2.2.0/Regexp.html