I suspect you are not a native English speaker. I'm pretty sure you don't mean "doubt" there, which means "I understand, but I do not agree". You almost certainly want "question", which means "I do not understand".
in reply to Regular Expression Doubt
I see this mistake frequently. I suspect it is because we can sometimes use "question" in place of "doubt", as in "I question the integrity of that bridge". But the opposite is never true.
Just putting this node here so I can refer to it in the future. Thanks for the opportunity.