Historically, the awk and sed regex engines had a \w that supported underscores and perl was designed as a much improved version of these tools. Don't know the history before that, but I suspect it is derived from the idea that C identifiers can contain letters, numbers and underscores. You can use Unicode character classes for alpha matching: 
$x =~ /^\p{IsAlpha}/;

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See perlretut for more examples.

-Mark

In reply to Re: Why does \w include numbers and underscore? by kvale
in thread Why does \w include numbers and underscore? by davido

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