#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)^0+?(\d{2})"
Local $sString = "001 - Filename 001.mp3" & @CRLF & _
"009 - Filename 009.mp3" & @CRLF & _
"010 - Filename 010.mp3" & @CRLF & _
"060 - Filename 060.mp3" & @CRLF & _
"086 - Filename 086.mp3" & @CRLF & _
"100 - Filename 100.mp3" & @CRLF & _
"1000 - Filename 1000.mp3" & @CRLF & _
"01001 - Filename 1001.mp3" & @CRLF & _
"" & @CRLF & _
"01 - Filename with two leading numbers.mp3" & @CRLF & _
"1 - Filename with one leading number.mp3"
Local $sSubst = "$1"
Local $sResult = StringRegExpReplace($sString, $sRegex, $sSubst)
MsgBox($MB_SYSTEMMODAL, "Result", $sResult)
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 AutoIt, please visit: https://www.autoitscript.com/autoit3/docs/functions/StringRegExp.htm