#include <MsgBoxConstants.au3> ; to declare the Constants of MsgBox
Local $sRegex = "(?m)[^b]:[^,]*,?"
Local $sString = "a:ABC,b:SDSD,c:213123#a:SDS,b:UIOU,c:89797#a:TYRQQQW,b:UIOUIOYYO,c:546654 a:DFSS,b:TYRTTN,c:12323#a:HJH,b:YTUUUTYUTYT,c:67890 a:TYY,b:OPIUIU,c:86768"
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