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Traditional Character Ranges
If we wanted to construct a regular expression that would find every file in our lists be - ginning with an uppercase letter, we could do this:
[me@linuxbox ~]$ grep -h '^[ABCDEFGHIJKLMNOPQRSTUVWXZY]' dirlist*.txt
[me@linuxbox ~]$ grep -h '^[ABCDEFGHIJKLMNOPQRSTUVWXZY]' dirlist*.txt
It’s just a matter of putting all 26 uppercase letters in a bracket expression. But the idea of all that typing is deeply troubling, so there is another way:
[me@linuxbox ~]$ grep -h '^[A-Z]' dirlist*.txt
MAKEDEV
ControlPanel GET
HEAD POST X X11
Xorg MAKEFLOPPIES
NetworkManager NetworkManagerDispatcher
[me@linuxbox ~]$ grep -h '^[A-Z]' dirlist*.txt
MAKEDEV
ControlPanel GET
HEAD POST X X11
Xorg MAKEFLOPPIES
NetworkManager NetworkManagerDispatcher
By using a three character range, we can abbreviate the 26 letters. Any range of charac -
ters can be expressed this way including multiple ranges, such as this expression that matches all filenames starting with letters and numbers:
[me@linuxbox ~]$ grep -h '^[A-Za-z0-9]' dirlist*.txt
[me@linuxbox ~]$ grep -h '^[A-Za-z0-9]' dirlist*.txt
In character ranges, we see that the dash character is treated specially, so how do we actu- ally include a dash character in a bracket expression? By making it the first character in the expression. Consider these two examples:
[me@linuxbox ~]$ grep -h '[A-Z]' dirlist*.txt
[me@linuxbox ~]$ grep -h '[A-Z]' dirlist*.txt
This will match every filename containing an uppercase letter. While:
[me@linuxbox ~]$ grep -h '[-AZ]' dirlist*.txt
[me@linuxbox ~]$ grep -h '[-AZ]' dirlist*.txt
will match every filename containing a dash, or an uppercase “A” or an uppercase “Z”.