Question

Please try

egrep "^[a-z]{3}$" /usr/share/dict/words

egrep "^[[:lower:]]{3}$" /usr/share/dict/words

The first one returns both uppercase and lowercase words.
The second one returns lowercase words only.

Answer 1

It has to do with your locale setting. If you set LC_ALL to C, it should work as expected.

From the egrep manpage under Ubuntu 11.04:

Within a bracket expression, a range expression consists of two characters separated by a hyphen. It matches any single character that sorts between the two characters, inclusive, using the locale’s collating sequence and character set.

For example, in the default C locale, [a-d] is equivalent to [abcd]. Many locales sort characters in dictionary order, and in these locales [a-d] is typically not equivalent to [abcd]; it might be equivalent to [aBbCcDd], for example. To obtain the traditional interpretation of bracket expressions, you can use the C locale by setting the LC_ALL environment variable to the value C.

You can try the commands from the following transcript to confirm this:

pax$ egrep "^[a-z]{3}$" /usr/share/dict/words | head -5l
AOL
Abe
Ada
Ala
Ali
pax$ LC_ALL=C egrep "^[a-z]{3}$" /usr/share/dict/words | head -5l
ace
act
add
ado
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