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Why is an underscore (_) not regarded as a non-word character? This regexp \W matches all non-word character but not the underscore.
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The underscore character, _, originally appeared on the typewriter and was primarily used to emphasise words as in the proofreader's convention.
Non-word characters include characters other than alphanumeric characters ( - , - and - ) and underscore (_).
Regex doesn't recognize underscore as special character.
The underscore ( _ ) is also known as an understrike, underbar, or underline, and is a character that was originally on a typewriter keyboard and was used simply to underline words or numbers for emphasis. Today, the character is used to create visual spacing in a sequence of words where whitespace is not permitted.
Referring to Jeffrey Friedl's book about Regular Expressions, this was a change in Perl Regular Expressions, originally. Back to 1988 according to characters that were allowed to name a Perl variable [Page 89]:
Perl 2 was released in June 1988. Larry had replaced the regex code entirely, this time using a greatly enhanced version of the Henry Spencer package mentioned in the previous section. You could still have at most nine sets of parentheses, but now you could use
|inside them. Support for\dand\swas added, and support for\wwas changed to include an underscore, since then it would match what characters were allowed in a Perl variable name.
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\W is defined as [^A-Za-z0-9_].
It is the opposite of \w which is [A-Za-z0-9_] and means "a word character".
It is not about words as you perceive them in a spoken language. The "word" here means an identifier. Most programming languages allow (uppercase and lowercase) letters, digit and underscores (_) in identifiers.
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