No output for $ grep '[^a-z]\{22\}' /usr/share/dict/words

Question

After executing

grep '[a-z]\{22\}' /usr/share/dict/words

the output is

counterrevolutionaries
electroencephalographs

but while executing

grep '[^a-z]\{22\}' /usr/share/dict/words

there is not output.

I was expecting that it will generate similar results as that of

grep -v '[a-z]\{22\}' /usr/share/dict/words

My question is what's wrong and why ?

Answer 1

[specification] matches a collating element (can be a character or sequence of characters as defined in the collating algorithm for the locale (for instance, in Hungarian locales on GNU systems, dzs is a collating element that sorts somewhere between d and e)) in the specified set.

That specification can include

• ranges like a-z (or [.dzs.]-z) for collating elements that collate between a and z (note that it generally includes abcdefghijklmnoprstuvwxyz but in most locales, it includes a lot more). Also, as POSIX leaves it unspecified for locales other than the POSIX locales, how much those ranges are based on the collation order varies significantly between implementations.
• individual characters or collating elements (x, [.dsz.])
• POSIX character classes [:alpha:], [:digit:]
• equivalent classes like [=e=] for all the collating elements that have the same primary collating weight as e (could include things like é)

So, for instance, [acd[=e=]h-k[:digit:][.dzs.]] matches on a collating element provided it's either a, c, d, dzs or is equivalent to e or collates between h and k or is classified as digit.

And if the specification starts with ^, then it still matches one collating element, but with the set complemented. That is any collating element but the ones specified.

So [^a-z] matches on any collating element that does not collate between a and z. For instance, it would probably match on 1 and ẑ, possibly on X or DSZ depending on the locale and the grep implementation, but not on a, x nor z and probably not on é.

So grep '[^a-z]\{22\}' matches on lines that contain a sequence of 22 collating elements that collate either before a or after z.

While grep -v '[a-z]\{22\}' matches on lines that do not contain a sequence of 22 collating elements collating between a and z.

Matching the same without -v is almost impossible to implement, you'd need to match on lines that contain no more than 21 [a-z] collating elements in between two [^a-z] element. But if the locale supports multi-character collating elements, that's not really possible. For instance, in those Hungarian locales, [a-z] matches on dsz but also on d, s, and z so you'll find that there, [a-z]{0,21} will match on dszxxxyyyxxxyyyxxxyyyx but also would [a-z]{22}.

For locales that don't have multi-character collating elements, you can do something like:

grep  '^[^a-z]*\([a-z]\{1,21\}[^a-z]\{1,\}\)*[a-z]\{0,21\}$'

Now, there are also some grep implementations that support more advanced regular syntaxes with options that have some negation operator.

For instance, the GNU or ast-open implementation of grep support perl-like (using libpcre in GNU grep, ast-open's own implementation for ast-open grep) regular expressions with the -P option which has a (?!pattern) negative look-ahead operator.

(?!pattern) matches with zero width at any point on the subject string provided the pattern doesn't match starting from there. So one could use:

 grep -P '^(?!.*[a-z]{22})'

to match on the start of the line provided it's not followed by any number of characters and 22 [a-z]s. Note however that in PCRE (not in ast-open), [a-z] only matches on abcdefghijklmnopqrstuvwxyz regardless of the locale.

ast-open also takes a -X option for what they call augmented regexps. Those augmented regex have a ! operator that negates things. x! would match on anything other than x (including the empty string).

So with ast-open grep, you could also do:

grep -X '^(.*[a-z]{22}.*)!$'

Answer 2

• grep '[^a-z]\{22\}' /usr/share/dict/words

  Looks for lines in the file /usr/share/dict/words which contain a string of 22 characters which are not lowercase letters. It is very likely that the file won't contain any such line. (Why would that file contain strings of 22 non-letters?)

• grep -v '[a-z]\{22\}' /usr/share/dict/words

  Looks for lines which do not contain strings of 22 letters. There will probably be very many such lines. (Because most words are shorter than 22 letters.)