If X is there I want everything after the X. If X is not there I want the whole string.

Try this on a copy of the string:

s/^.*?X//;
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This will only substitute everything upto the first X, if one exists. It'll not change the string if it doesn't.

Drop the question mark if you want to locate the last "X".

And if the string can contain newlines, add the /s modifier, which changes the matching behaviour of /./ to possibly match a newline as well.