Re^4: regex at word boundary

(?!) explained:

From perlre: "A zero-width negative look-ahead assertion. For example /foo(?!bar)/ matches any occurrence of "foo" that isn't followed by "bar"." So in other words, (?!) is a negative lookahead assertion that "must not match nothing" (an impossibility).

YAPE::Regex::Explain describes it simply like this:

perl -MYAPE::Regex::Explain -e "printYAPE::Regex::Explain->new( qr/(?!
+)/ )->explain();"

The regular expression:

(?-imsx:(?!))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?!)                     fail
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------
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Dave

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Re^5: regex at word boundary by QM (Parson) on Dec 08, 2005 at 05:12 UTC
"must not match nothing" I understand that at some level, but my brain is yelling "double negative!!". It might be better to say: `(?!)` is the negative look-ahead assertion for the null string. Since the null string matches anything and everything (including nothing), `(?!)` always fails. [I was tempted to find a symbolic representation for the null string, but I thougt that would only confuse the issue.] -QM -- Quantum Mechanics: The dreams stuff is made of	[reply] [d/l] [select]

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Re^5: regex at word boundary
by QM (Parson) on Dec 08, 2005 at 05:12 UTC

"must not match nothing"

(?!) is the negative look-ahead assertion for the null string. Since the null string matches anything and everything (including nothing), (?!) always fails.

-QM
--
Quantum Mechanics: The dreams stuff is made of

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