grep, global regular expression print

Literal

• They match the pattern literally, character-for-character.
• Unless modified by other expression mechanisms.
  -> Helpful to think as matching string of characters
  -> rather than matching a word.
  Becomes important distinction as you learn more complex patterns.

Myquestion: What is an expression mechanisms?

Lines that match GNU at the very beginning of the line.
Use ^ anchor before the literal string.

grep "^GNU" GPL-3
Copy

$ match be valid if occurs at end of line.
Command will match every line ending with the word and in the GPL-3 file:

grep "and$" GPL-3
Copy

Conclusion: Very similar as how VIM uses the commands as well

The period character (.) is used in regular expressions to mean that any single character can exist at the specified location.

For example, to match anything in the GPL-3 file that has two characters and then the string cept, you would use the following pattern:
grep "..cept" GPL-3

Output

use, which is precisely where it is most unacceptable.  Therefore, we
infringement under applicable copyright law, except executing it on a
tells the user that there is no warranty for the work (except to the
License by making exceptions from one or more of its conditions.
form of a separately written license, or stated as exceptions;
  You may not propagate or modify a covered work except as expressly
  9. Acceptance Not Required for Having Copies.
Copy

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
Copy

Outputyour 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:
Copy

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
Copy

Due to some legacy sorting issues,
often accurate to use POSIX character classes instead of character ranges

Example of POSIX Character classes
An example that accomplish the same procedure as the previous example uses the \[:upper:\] character class within a bracket selector:

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

*, which means “repeat the previous character or expression zero or more times”.
To find each line in the GPL-3 file that contains an opening and closing parenthesis, with only letters and single spaces in between, use the following expression:

grep "([A-Za-z ]*)" GPL-3
Copy

Where you’ll need to search for a literal period or opening bracket,
especially when working with source code or configuration files.
Because these characters have special meaning in regular expressions,
you need to “escape” these characters to tell grep that you do not wish to use their special meaning in this case.

You escape characters by using the backslash character (\) in front of the character that would normally have a special meaning.

For instance, to find any line that begins with a capital letter and ends with a period, use the following expression which escapes the ending period so that it represents a literal period instead of the usual “any character” meaning:

grep "^[A-Z].*\.$" GPL-3
Copy

One of the most useful abilities that extended regular expressions open up is the ability to group expressions together to manipulate or reference as one unit.

To group expressions together, wrap them in parentheses. If you would like to use parentheses without using extended regular expressions, you can escape them with the backslash to enable this functionality. This means that the following three expressions are functionally equivalent:

grep "\(grouping\)" file.txt
grep -E "(grouping)" file.txt
egrep "(grouping)" file.txt
Copy

To indicate alternation, use the pipe character |. These are often used within parenthetical grouping to specify that one of two or more possibilities should be considered a match.

The following will find either GPL or General Public License in the text:

grep -E "(GPL|General Public License)" GPL-3
Copy

?? Like an or boolean expression?

Alternation can select between more than two choices by adding additional choices within the selection group separated by additional pipe (|) characters.

To match a character zero or one times, you can use the ? character. This makes character or character sets that came before optional, in essence.

The following matches copyright and right by putting copy in an optional group:

grep -E "(copy)?right" GPL-3
Copy

The + character matches an expression one or more times. This is almost like the * meta-character, but with the + character, the expression must match at least once.

The following expression matches the string free plus one or more characters that are not white space characters:

grep -E "free[^[:space:]]+" GPL-3
Copy

To specify the number of times that a match is repeated, use the brace characters ({ and }). These characters let you specify an exact number, a range, or an upper or lower bounds to the amount of times an expression can match.

Use the following expression to find all of the lines in the GPL-3 file that contain triple-vowels:

grep -E "[AEIOUaeiou]{3}" GPL-3
Copy

To match any words that have between 16 and 20 characters, use the following expression:

grep -E "[[:alpha:]]{16,20}" GPL-3
Copy

1. https://www.digitalocean.com/community/tutorials/using-grep-regular-expressions-to-search-for-text-patterns-in-linux