Extracting "true" words

[candide]

Back again with my study of regular expressions ;) There exists a 
special character allowing alphanumeric extraction, the special 
character \w (BTW, what the letter 'w' refers to?). But this feature 
doesn't permit to extract true words; by "true" I mean word composed 
only of _alphabetic_ letters (not digit nor underscore).

So I was wondering what is the pattern to extract (or to match) _true_ 
words ? Of course, I don't restrict myself to the ascii universe so that 
the pattern [a-zA-Z]+ doesn't meet my needs.
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