Your first example wouldn't match a line that began with XY, because it requires some character to precede it. Negative lookbehind or an alternation would fix: 
/(?:^|\S)XY/
# or
/(?<!\s)XY/

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(I also prefer the builtin not-whitespace char, rather than negating the class).
Caution: Contents may have been coded under pressure.

In reply to Re^2: Negating a leading whitespace by Roy Johnson
in thread Negating a leading whitespace by icg

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