Regular expressions are patterns that only certain commands are able to interpret. Regular expressions can be expanded to match certain sequences of characters in text. The examples displayed on this page will make use of regular expressions to demonstrate their power when used with the
grep command. In addition, these examples provide a very visual demonstration of how regular expressions work, the text that matches will be displayed in a red color.Follow Along
Use the following cd command to change to the Documents directory.
sysadmin@localhost:~$ cd ~/Documents
The simplest of all regular expressions use only literal characters, like the example from the previous page:
sysadmin@localhost:~/Documents$ grep sysadmin passwd sysadmin:x:1001:1001:System Administrator,,,,:/home/sysadmin:/bin/bash
Anchor Characters
Anchor characters are one of the ways regular expressions can be used to narrow down search results. For example, the pattern root appears many times in the /etc/passwd file:
sysadmin@localhost:~/Documents$ grep 'root' passwd root:x:0:0:root:/root:/bin/bash operator:x:1000:37::/root:
To prevent the shell from misinterpreting them as special shell characters, these patterns should be protected by strong quotes, which simply means placing them between single quotes.
The first anchor character ^ is used to ensure that a pattern appears at the beginning of the line. For example, to find all lines in /etc/passwd that start with root use the pattern ^root. Note that ^ must be the first character in the pattern to be effective.
sysadmin@localhost:~/Documents$ grep '^root' /etc/passwd root:x:0:0:root:/root:/bin/bash
For the next example, first examine the alpha-first.txt file. The cat command can be used to print the contents of a file:
sysadmin@localhost:~/Documents$ cat alpha-first.txt A is for Animal B is for Bear C is for Cat D is for Dog E is for Elephant F is for Flower
The second anchor character $ can be used to ensure a pattern appears at the end of the line, thereby effectively reducing the search results. To find the lines that end with an r in the alpha-first.txt file, use the pattern r$:
sysadmin@localhost:~/Documents$ grep 'r$' alpha-first.txt B is for Bear F is for Flower
Again, the position of this character is important, the $ must be the last character in the pattern in order to be effective as an anchor.
Match a Single Character With .
The following examples will use the red.txt file:
sysadmin@localhost:~/Documents$ cat red.txt red reef rot reeed rd rod roof reed root reel read
One of the most useful expressions is the period . character. It will match any character except for the new line character. The pattern r..f would find any line that contained the letter r followed by exactly two characters (which can be any character except a newline) and then the letter f:
sysadmin@localhost:~/Documents$ grep 'r..f' red.txt reef roof
The same concept can be repeated using other combinations. The following will find four letter words that start with r and with d:
sysadmin@localhost:~/Documents$ grep 'r..d' red.txt reed read
This character can be used any number of times. To find all words that have at least four characters the following pattern can be used:
sysadmin@localhost:~/Documents$ grep '....' red.txt reef reeed roof reed root reel read
The line does not have to be an exact match, it simply must contain the pattern, as seen here when r..t is searched for in the /etc/passwd file:
sysadmin@localhost:~/Documents$ grep 'r..t' /etc/passwd root:x:0:0:root:/root:/bin/bash operator:x:1000:37::/root:
Match a Single Character With []
The square brackets [ ] match a single character from the list or range of possible characters contained within the brackets.
For example, given the profile.txt file:
sysadmin@localhost:~/Documents$ cat profile.txt Hello my name is Joe. I am 37 years old. 3121991 My favorite food is avocados. I have 2 dogs. 123456789101112
To find all the lines in the profile.txt which have a number in them, use the pattern [0123456789] or [0-9]:
sysadmin@localhost:~/Documents$ grep '[0-9]' profile.txt I am 37 years old. 3121991 I have 2 dogs. 123456789101112
On the other hand, to find all the lines which contain any non-numeric characters, insert a ^ as the first character inside the brackets. This character negates the characters listed:
sysadmin@localhost:~/Documents$ grep '[^0-9]' profile.txt Hello my name is Joe. I am 37 years old. My favorite food is avocados. I have 2 dogs.
Note
Do not mistake [^0-9] to match lines which do not contain numbers. It actually matches lines which contain non-numbers. Look at the original file to see the difference. The third and sixth lines only contain numbers, they do not contain non-numbers so those lines do not match.
When other regular expression characters are placed inside of square brackets, they are treated as literal characters. For example, the . normally matches any one character, but placed inside the square brackets, then it will just match itself. In the next example, only lines which contain the . character are matched.
sysadmin@localhost:~/Documents$ grep '[.]' profile.txt Hello my name is Joe. I am 37 years old. My favorite food is avocados. I have 2 dogs.
Match a Repeated Character Or Patterns With *
The regular expression character * is used to match zero or more occurrences of a character or pattern preceding it. For example e* would match zero or more occurrences of the letter e:
sysadmin@localhost:~/Documents$ cat red.txt red reef rot reeed rd rod roof reed root reel read sysadmin@localhost:~/Documents$ grep 're*d' red.txt red reeed rd reed
It is also possible to match zero or more occurrences of a list of characters by utilizing the square brackets. The pattern [oe]* used in the following example will match zero or more occurrences of the o character or the e character:
sysadmin@localhost:~/Documents$ grep 'r[oe]*d' red.txt red reeed rd rod reed
When used with only one other character, * isn't very helpful. Any of the following patterns would match every string or line in the file: .* e* b* z*.
sysadmin@localhost:~/Documents$ grep 'z*' red.txt red reef rot reeed rd rod roof reed root reel read
sysadmin@localhost:~/Documents$ grep 'e*' red.txt red reef rot reeed rd rod roof reed root reel read
This is because * can match zero occurrences of a pattern. In order to make the * useful, it is necessary to create a pattern which includes more than just the one character preceding *. For example, the results above can be refined by adding another e to make the pattern ee* effectively matching every line which contains at least one e.
sysadmin@localhost:~/Documents$ grep 'ee*' red.txt red reef reeed reed reel read
Standard Input
If a file name is not given, the grep command will read from standard input, which normally comes from the keyboard with input provided by the user who runs the command. This provides an interactive experience with grep where the user types in the input and grep filters as it goes. Feel free to try it out, just press Ctrl-D when you're ready to return to the prompt.
Follow Along
Use the following cd command to return to the home directory:
sysadmin@localhost:~/Documents$ cd ~
