#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)^(\w+): (.*)\n((?:(?!\1).*\n)+)\1: (.*\n)"
Local $sString = "abc: bla1 bla1 bla1..." & @CRLF & _
"cde: bla bla bla..." & @CRLF & _
"ghk: bla1 bla1 bla1..." & @CRLF & _
"lmn: bla bla bla..." & @CRLF & _
"abc: bla2 bla2 bla2..." & @CRLF & _
"bcd: bla bla bla..." & @CRLF & _
"ghk: bla2 bla2 bla2..." & @CRLF & _
"xyz: bla bla bla..."
Local $sSubst = "\1: \2 \4\3"
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