#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?i)(?::|^)((?:0+:){2,}0+)(?=:|$)|(?::|^)(0+)([0-9a-f]+)(?=:|$)|\b0+([0-9a-f]{1,4})\b"
Local $sString = "2001:0db8:abcd:0012:0000:0000:0000:0001" & @CRLF & _
"2001:0db8:abcd:0012:0000:0000:0000:00a2" & @CRLF & _
"2001:0db8:abcd:0012:0000:0000:0000:00ff" & @CRLF & _
"a:b:0:c:d:e:f:g" & @CRLF & _
"2001:0db8:abcd:0012:0000:0000:0000:0001" & @CRLF & _
":00ff:0000:" & @CRLF & _
"0012:0000:0000:0000:0001"
Local $sSubst = "${1:+::}${3}${4}"
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