#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)\.zip\b[^,]*(*SKIP)(*F)|\s"
Local $sString = "http://abcd.com/old file/anotherfile.zip/some contents/sample.txt,http://abcd.com/new file/anotherfile.zip/some more contents/new sample.txt,http://abcd.com/newer file/another file.zip/some more contents/old sample.txt,http://abcd.com/newer file/another file.txt" & @CRLF & _
""
Local $sSubst = "%20"
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