How do I use the Grep command with regular expressions on a Linux and Unix-like operating systems?
Linux comes with GNU grep, which supports extended regular expressions. GNU grep is the default on all Linux systems. The grep command is used to locate information stored anywhere on your server or workstation.
### Regular Expressions ###
Regular Expressions is nothing but a pattern to match for each input line. A pattern is a sequence of characters. Following all are examples of pattern:
Search vivek in any case (i.e. case insensitive search)
grep -i -w vivek /etc/passwd
Search vivek or raj in any case
grep -E -i -w 'vivek|raj' /etc/passwd
The PATTERN in last example, used as an extended regular expression.
### Anchors ###
You can use ^ and $ to force a regex to match only at the start or end of a line, respectively. The following example displays lines starting with the vivek only:
You can display only lines starting with the word vivek only i.e. do not display vivekgite, vivekg etc:
grep -w ^vivek /etc/passwd
Find lines ending with word foo:
grep 'foo$' filename
Match line only containing foo:
grep '^foo$' filename
You can search for blank lines with the following examples:
grep '^$' filename
### Character Class ###
Match Vivek or vivek:
grep '[vV]ivek' filename
OR
grep '[vV][iI][Vv][Ee][kK]' filename
You can also match digits (i.e match vivek1 or Vivek2 etc):
grep -w '[vV]ivek[0-9]' filename
You can match two numeric digits (i.e. match foo11, foo12 etc):
grep 'foo[0-9][0-9]' filename
You are not limited to digits, you can match at least one letter:
grep '[A-Za-z]' filename
Display all the lines containing either a "w" or "n" character:
grep [wn] filename
Within a bracket expression, the name of a character class enclosed in "[:" and ":]" stands for the list of all characters belonging to that class. Standard character class names are:
- [:alnum:] - Alphanumeric characters.
- [:alpha:] - Alphabetic characters
- [:blank:] - Blank characters: space and tab.
- [:digit:] - Digits: '0 1 2 3 4 5 6 7 8 9'.
- [:lower:] - Lower-case letters: 'a b c d e f g h i j k l m n o p q r s t u v w x y z'.
- [:space:] - Space characters: tab, newline, vertical tab, form feed, carriage return, and space.
- [:upper:] - Upper-case letters: 'A B C D E F G H I J K L M N O P Q R S T U V W X Y Z'.
In this example match all upper case letters:
grep '[:upper:]' filename
### Wildcards ###
You can use the "." for a single character match. In this example match all 3 character word starting with "b" and ending in "t":
grep '\<b.t\>' filename
Where,
- \< Match the empty string at the beginning of word
- \> Match the empty string at the end of word.
Print all lines with exactly two characters:
grep '^..$' filename
Display any lines starting with a dot and digit:
grep '^\.[0-9]' filename
#### Escaping the dot ####
The following regex to find an IP address 192.168.1.254 will not work:
grep '192.168.1.254' /etc/hosts
All three dots need to be escaped:
grep '192\.168\.1\.254' /etc/hosts
The following example will only match an IP address:
