Re^4: Quick Regex Question

in reply to Re^3: Quick Regex Question
in thread Quick Regex Question

... using a zero-width negative look-ahead assertion; or ... using a z +ero-width positive look-ahead assertion together with \D ...
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Note that (?!\d) and (?=\D) are not ~~complementary~~equivalent. The (?!\d) assertion is satisfied by having any non-digit follow it or by nothing, i.e., by the end of the string, nothing being not-a-digit. The (?=\D) assertion must be followed by a non-digit character. And similarly with the (?<!\d) and (?<=\D) look-behinds.

>perl -wMstrict -le
"my $s = 'xyzINC1234567';
 ;;
 print 'negative assertion true' if $s =~ /INC\d{7}(?!\d)/;
 print 'positive assertion true' if $s =~ /INC\d{7}(?=\D)/;
"
negative assertion true
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In this behavior, (?!\d) and, in general, (?!character-or-character-class) and its negative look-behind cousin are similar to (but not exactly the same as) the \b assertion, which may be true at the beginning or end of a string.

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