Grep Bracket Expressions

 

By placing a group of characters within brackets (\[ and \]), you can specify that the character at that position can be any one character found within the bracket group.

For example, to find the lines that contain too or two, you would specify those variations succinctly by using the following pattern:

grep "t[wo]o" GPL-3

 

The output shows that both variations exist in the file:

Output
your programs, too.
freedoms that you received.  You must make sure that they, too, receive
  Developers that use the GNU GPL protect your rights with two steps:
a computer network, with no transfer of a copy, is not conveying.
System Libraries, or general-purpose tools or generally available free
    Corresponding Source from a network server at no charge.
...
...

Bracket notation gives you some interesting options. You can have the pattern match anything except the characters within a bracket by beginning the list of characters within the brackets with a ^ character.

This example is like the pattern .ode, but will not match the pattern code:

grep "[^c]ode" GPL-3

 

Here’s the output you’ll see:

Output
  1. Source Code.
    model, to give anyone who possesses the object code either (1) a
the only significant mode of use of the product.
notice like this when it starts in an interactive mode:

Notice that in the second line returned, there is, in fact, the word code. This is not a failure of the regular expression or grep. Rather, this line was returned because earlier in the line, the pattern mode, found within the word model, was found. The line was returned because there was an instance that matched the pattern.

Another helpful feature of brackets is that you can specify a range of characters instead of individually typing every available character.

This means that if you want to find every line that begins with a capital letter, you can use the following pattern:

grep "^[A-Z]" GPL-3

 

Here’s the output you’ll see:

Output
GNU General Public License for most of our software; it applies also to
States should not allow patents to restrict development and use of
License.  Each licensee is addressed as "you".  "Licensees" and
Component, and (b) serves only to enable use of the work with that
Major Component, or to implement a Standard Interface for which an
System Libraries, or general-purpose tools or generally available free
Source.
User Product is transferred to the recipient in perpetuity or for a
...
...

Due to some legacy sorting issues, it is often more accurate to use POSIX character classes instead of character ranges like you just used.

There are many character classes that are outside of the scope of this guide, but an example that would accomplish the same procedure as the previous example uses the \[:upper:\] character class within a bracket selector:

grep "^[[:upper:]]" GPL-3

 

The output will be the same as before.