Use looking behind -
$string =~ /((?<=X)\S+)/;

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Note that this only sets $1 if X is present.

Can you please phrase your question more clearly, ie. what's the behaviour you are expecting? What do you want to capture when X is not present?

In reply to Re: Capturing everything after an optional character in a regex? by Roger
in thread Capturing everything after an optional character in a regex? by Anonymous Monk

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