#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "#[0-9a-fA-F]{6,6};"
Local $sString = "<a href="//10.1.1.175/Intranetv2/content_kostenstellen.php" style="margin-top:10px; background-color:#800000; border-color:#800000;" class="btn btn-warning">Kostenstellen</a>"
Local $sSubst = ""
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