Unix / Linux - Regular Expressions with SED

In this chapter, we will discuss in detail about regular expressions with SED in Unix.
A regular expression is a string that can be used to describe several sequences of characters. Regular expressions are used by several different Unix commands, including ed, sed, awk, grep, and to a more limited extent, vi.
Here SED stands for stream editor. This stream-oriented editor was created exclusively for executing scripts. Thus, all the input you feed into it passes through and goes to STDOUT and it does not change the input file.
Before we start, let us ensure we have a local copy of /etc/passwd text file to work with sed.
As mentioned previously, sed can be invoked by sending data through a pipe to it as follows −
$ cat /etc/passwd | sed
Usage: sed [OPTION]... {script-other-script} [input-file]...

  -n, --quiet, --silent
                 suppress automatic printing of pattern space
  -e script, --expression = script
...............................

The cat command dumps the contents of /etc/passwd to sed through the pipe into sed's pattern space. The pattern space is the internal work buffer that sed uses for its operations.
Following is the general syntax for sed −
/pattern/action

Here, pattern is a regular expression, and action is one of the commands given in the following table. If pattern is omitted, action is performed for every line as we have seen above.
The slash character (/) that surrounds the pattern are required because they are used as delimiters.
p
Prints the line
d
Deletes the line
s/pattern1/pattern2/
Substitutes the first occurrence of pattern1 with pattern2
We will now understand how to delete all lines with sed. Invoke sed again; but the sed is now supposed to use the editing command delete line, denoted by the single letter d −
$ cat /etc/passwd | sed 'd'
$

Instead of invoking sed by sending a file to it through a pipe, the sed can be instructed to read the data from a file, as in the following example.
The following command does exactly the same as in the previous example, without the cat command −
$ sed -e 'd' /etc/passwd
$

The sed also supports addresses. Addresses are either particular locations in a file or a range where a particular editing command should be applied. When the sed encounters no addresses, it performs its operations on every line in the file.
The following command adds a basic address to the sed command you've been using −
$ cat /etc/passwd | sed '1d' |more
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
$

Notice that the number 1 is added before the delete edit command. This instructs the sed to perform the editing command on the first line of the file. In this example, the sed will delete the first line of /etc/password and print the rest of the file.
We will now understand how to work with the sed address ranges. So what if you want to remove more than one line from a file? You can specify an address range with sed as follows −
$ cat /etc/passwd | sed '1, 5d' |more
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
$

The above command will be applied on all the lines starting from 1 through 5. This deletes the first five lines.
Try out the following address ranges −
'4,10d'
Lines starting from the 4th till the 10th are deleted
'10,4d'
Only 10th line is deleted, because the sed does not work in reverse direction
'4,+5d'
This matches line 4 in the file, deletes that line, continues to delete the next five lines, and then ceases its deletion and prints the rest
'2,5!d'
This deletes everything except starting from 2nd till 5th line
'1~3d'
This deletes the first line, steps over the next three lines, and then deletes the fourth line. Sed continues to apply this pattern until the end of the file.
'2~2d'
This tells sed to delete the second line, step over the next line, delete the next line, and repeat until the end of the file is reached
'4,10p'
Lines starting from 4th till 10th are printed
'4,d'
This generates the syntax error
',10d'
This would also generate syntax error
Note − While using the p action, you should use the -n option to avoid repetition of line printing. Check the difference in between the following two commands −
$ cat /etc/passwd | sed -n '1,3p'
Check the above command without -n as follows −
$ cat /etc/passwd | sed '1,3p'

The substitution command, denoted by s, will substitute any string that you specify with any other string that you specify.
To substitute one string with another, the sed needs to have the information on where the first string ends and the substitution string begins. For this, we proceed with bookending the two strings with the forward slash (/) character.
The following command substitutes the first occurrence on a line of the string root with the string amrood.
$ cat /etc/passwd | sed 's/root/amrood/'
amrood:x:0:0:root user:/root:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
..........................

It is very important to note that sed substitutes only the first occurrence on a line. If the string root occurs more than once on a line only the first match will be replaced.
For the sed to perform a global substitution, add the letter g to the end of the command as follows −
$ cat /etc/passwd | sed 's/root/amrood/g'
amrood:x:0:0:amrood user:/amrood:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
...........................

There are a number of other useful flags that can be passed in addition to the g flag, and you can specify more than one at a time.
g
Replaces all matches, not just the first match
NUMBER
Replaces only NUMBERth match
p
If substitution was made, then prints the pattern space
w FILENAME
If substitution was made, then writes result to FILENAME
I or i
Matches in a case-insensitive manner
M or m
In addition to the normal behavior of the special regular expression characters ^ and $, this flag causes ^ to match the empty string after a newline and $ to match the empty string before a newline
Suppose you have to do a substitution on a string that includes the forward slash character. In this case, you can specify a different separator by providing the designated character after the s.
$ cat /etc/passwd | sed 's:/root:/amrood:g'
amrood:x:0:0:amrood user:/amrood:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh

In the above example, we have used : as the delimiter instead of slash / because we were trying to search /root instead of the simple root.
Use an empty substitution string to delete the root string from the /etc/passwd file entirely −
$ cat /etc/passwd | sed 's/root//g'
:x:0:0::/:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh

If you want to substitute the string sh with the string quiet only on line 10, you can specify it as follows −
$ cat /etc/passwd | sed '10s/sh/quiet/g'
root:x:0:0:root user:/root:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/quiet

Similarly, to do an address range substitution, you could do something like the following −
$ cat /etc/passwd | sed '1,5s/sh/quiet/g'
root:x:0:0:root user:/root:/bin/quiet
daemon:x:1:1:daemon:/usr/sbin:/bin/quiet
bin:x:2:2:bin:/bin:/bin/quiet
sys:x:3:3:sys:/dev:/bin/quiet
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh

As you can see from the output, the first five lines had the string sh changed to quiet, but the rest of the lines were left untouched.
You would use the p option along with the -n option to print all the matching lines as follows −
$ cat testing | sed -n '/root/p'
root:x:0:0:root user:/root:/bin/sh
[root@ip-72-167-112-17 amrood]# vi testing
root:x:0:0:root user:/root:/bin/sh
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh

While matching patterns, you can use the regular expression which provides more flexibility.
Check the following example which matches all the lines starting with daemon and then deletes them −
$ cat testing | sed '/^daemon/d'
root:x:0:0:root user:/root:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh

Following is the example which deletes all the lines ending with sh −
$ cat testing | sed '/sh$/d'
sync:x:4:65534:sync:/bin:/bin/sync

The following table lists four special characters that are very useful in regular expressions.
^
Matches the beginning of lines
$
Matches the end of lines
.
Matches any single character
*
Matches zero or more occurrences of the previous character
[chars]
Matches any one of the characters given in chars, where chars is a sequence of characters. You can use the - character to indicate a range of characters.
Look at a few more expressions to demonstrate the use of metacharacters. For example, the following pattern −
/a.c/
Matches lines that contain strings such as a+c, a-c, abc, match, and a3c
/a*c/
Matches the same strings along with strings such as ace, yacc, and arctic
/[tT]he/
Matches the string The and the
/^$/
Matches blank lines
/^.*$/
Matches an entire line whatever it is
/ */
Matches one or more spaces
/^$/
Matches blank lines
Following table shows some frequently used sets of characters −
[a-z]
Matches a single lowercase letter
[A-Z]
Matches a single uppercase letter
[a-zA-Z]
Matches a single letter
[0-9]
Matches a single number
[a-zA-Z0-9]
Matches a single letter or number
Some special keywords are commonly available to regexps, especially GNU utilities that employ regexps. These are very useful for sed regular expressions as they simplify things and enhance readability.
For example, the characters a through z and the characters A through Z, constitute one such class of characters that has the keyword [[:alpha:]]
Using the alphabet character class keyword, this command prints only those lines in the /etc/syslog.conf file that start with a letter of the alphabet −
$ cat /etc/syslog.conf | sed -n '/^[[:alpha:]]/p'
authpriv.*                         /var/log/secure
mail.*                             -/var/log/maillog
cron.*                             /var/log/cron
uucp,news.crit                     /var/log/spooler
local7.*                           /var/log/boot.log

The following table is a complete list of the available character class keywords in GNU sed.
[[:alnum:]]
Alphanumeric [a-z A-Z 0-9]
[[:alpha:]]
Alphabetic [a-z A-Z]
[[:blank:]]
Blank characters (spaces or tabs)
[[:cntrl:]]
Control characters
[[:digit:]]
Numbers [0-9]
[[:graph:]]
Any visible characters (excludes whitespace)
[[:lower:]]
Lowercase letters [a-z]
[[:print:]]
Printable characters (non-control characters)
[[:punct:]]
Punctuation characters
[[:space:]]
Whitespace
[[:upper:]]
Uppercase letters [A-Z]
[[:xdigit:]]
Hex digits [0-9 a-f A-F]
The sed metacharacter & represents the contents of the pattern that was matched. For instance, say you have a file called phone.txt full of phone numbers, such as the following −
5555551212
5555551213
5555551214
6665551215
6665551216
7775551217

You want to make the area code (the first three digits) surrounded by parentheses for easier reading. To do this, you can use the ampersand replacement character −
$ sed -e 's/^[[:digit:]][[:digit:]][[:digit:]]/(&)/g' phone.txt
(555)5551212
(555)5551213
(555)5551214
(666)5551215

(666)5551216
(777)5551217

Here in the pattern part you are matching the first 3 digits and then using & you are replacing those 3 digits with the surrounding parentheses.
You can use multiple sed commands in a single sed command as follows −
$ sed -e 'command1' -e 'command2' ... -e 'commandN' files

Here command1 through commandN are sed commands of the type discussed previously. These commands are applied to each of the lines in the list of files given by files.
Using the same mechanism, we can write the above phone number example as follows −
$ sed -e 's/^[[:digit:]]\{3\}/(&)/g'  \ 
   -e 's/)[[:digit:]]\{3\}/&-/g' phone.txt 
(555)555-1212 
(555)555-1213 
(555)555-1214 
(666)555-1215 
(666)555-1216 
(777)555-1217

Note − In the above example, instead of repeating the character class keyword [[:digit:]] three times, we replaced it with \{3\}, which means the preceding regular expression is matched three times. We have also used \ to give line break and this has to be removed before the command is run.
The ampersand metacharacter is useful, but even more useful is the ability to define specific regions in regular expressions. These special regions can be used as reference in your replacement strings. By defining specific parts of a regular expression, you can then refer back to those parts with a special reference character.
To do back references, you have to first define a region and then refer back to that region. To define a region, you insert backslashed parentheses around each region of interest. The first region that you surround with backslashes is then referenced by \1, the second region by \2, and so on.
Assuming phone.txt has the following text −
(555)555-1212
(555)555-1213
(555)555-1214
(666)555-1215
(666)555-1216
(777)555-1217

Try the following command −
$ cat phone.txt | sed 's/\(.*)\)\(.*-\)\(.*$\)/Area \ 
   code: \1 Second: \2 Third: \3/' 
Area code: (555) Second: 555- Third: 1212 
Area code: (555) Second: 555- Third: 1213 
Area code: (555) Second: 555- Third: 1214 
Area code: (666) Second: 555- Third: 1215 
Area code: (666) Second: 555- Third: 1216 
Area code: (777) Second: 555- Third: 1217

Note − In the above example, each regular expression inside the parenthesis would be back referenced by \1, \2 and so on. We have used \ to give line break here. This should be removed before running the command.

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