is a word boundary.
So,
is similar to [^a-zA-Z0-9_]
i.e
would check for anything except word
You can instead use this regex
(?<=s|^)[a-zA-Z]+(?=s|$)
-------- --------- ------
| | |->match only if the pattern is followed by a space(s) or end of string/line($)
| |->pattern
|->match only if the pattern is preceded by space(s) or start of stringline(^)
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