#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?mi)\{\{(?:(?!\}\}).)*?default=(.*?)\}\}"
Local $sString = "It was a {{color, default=black}} and scary {{phase}}. As the {{animals, default=dogs}} {{make_sound, default=barked}} and the trees swayed {{setting, default=to the breeze}}, a {{size, default=}} {{monster, default=troll}} that emerged from the shadows."
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