Re: Question about regex with negated character class

Kaz Kylheku <>
Mon, 25 Apr 2022 23:46:44 -0000 (UTC)

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Question about regex with negated character class (Roger L Costello) (2022-04-25)
Re: Question about regex with negated character class (Kaz Kylheku) (2022-04-25)
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From: Kaz Kylheku <>
Newsgroups: comp.compilers
Date: Mon, 25 Apr 2022 23:46:44 -0000 (UTC)
Organization: A noiseless patient Spider
References: 22-04-015
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Keywords: lex
Posted-Date: 25 Apr 2022 22:40:32 EDT

On 2022-04-25, Roger L Costello <> wrote:
> Hi Folks,
> On page 12 of the Flex specification it says this:
> "A negated character class such as [^A-Z] will match a newline
> unless \n (or an equivalent escape sequence) is one of the characters
> explicitly present
> in the negated character class (e.g., [^A-Z\n]). This is unlike how many other
> regular expression tools treat negated character classes ..."

I suspect this is a documentation mistake (in terms of the the remark it
makes about other regex implementations).

There is something special in Flex with regard to newlines: namely the
any-character regular expression . (dot) does not match any character:
it excludes the newline. The documenter might have momentarily gotten
their wires crossed, misremembering what is the special behavior.

Or else, I also agree with John that it may in fact be a remark about
regex implementations in line-oriented text processing utilities, which
(in their standrad forms, e.g. POSIX) don't have multi-line matching
features in which \n appears as a character.

TXR Programming Language:
Cygnal: Cygwin Native Application Library:

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