#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)(?:^```\h*\S+\s+\K|\G(?!\A))^(?!```)(.*\R+)(?:```)?"
Local $sString = "Do not change this line" & @CRLF & _
"" & @CRLF & _
"```markdown" & @CRLF & _
"Sasha great!" & @CRLF & _
" Sasha nice!" & @CRLF & _
"She is beautiful, surprise!" & @CRLF & _
"```" & @CRLF & _
"Do not change this line" & @CRLF & _
"" & @CRLF & _
"Do not change this line" & @CRLF & _
"" & @CRLF & _
"```markdown" & @CRLF & _
"Sasha great!" & @CRLF & _
" Sasha nice!" & @CRLF & _
"She is beautiful, surprise!" & @CRLF & _
"```" & @CRLF & _
"" & @CRLF & _
"Do not change this line" & @CRLF & _
"" & @CRLF & _
"" & @CRLF & _
"Do not change this line" & @CRLF & _
"" & @CRLF & _
"```markdown" & @CRLF & _
"Sasha great!" & @CRLF & _
" Sasha nice!" & @CRLF & _
"She is beautiful, surprise!" & @CRLF & _
"```" & @CRLF & _
"" & @CRLF & _
"Do not change this line" & @CRLF & _
"" & @CRLF & _
"" & @CRLF & _
"Do not change this line" & @CRLF & _
"" & @CRLF & _
"```markdown" & @CRLF & _
"Sasha great!" & @CRLF & _
" Sasha nice!" & @CRLF & _
"She is beautiful, surprise!" & @CRLF & _
"```" & @CRLF & _
"" & @CRLF & _
"Do not change this line"
Local $sSubst = "\t\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