Character classes

Character classes, also known as a character set, is a regular expression consisting of a group of characters enclosed in brackets. It matches any one of the enclosed characters, irrespective of the order of occurrence of that character in the bracket. For example, [abcdefgh] matches any of the a, b, c, d, e, f, g, or h characters. 

In the following example, we print the employee information for those whose names begin with any of the characters enclosed within brackets:

$ awk '/^[ ABCDEFGHIJ ]/{ print }' emp.dat

The output on execution of the given code is as follows:

Jack    Singh   9857532312  jack@gmail.com      M   hr      2000
Jane    Kaur    9837432312  jane@gmail.com      F   hr      1800
Eva     Chabra  8827232115  eva@gmail.com       F   lgs     2100
Amit    Sharma  9911887766  amit@yahoo.com      M   lgs     2350
Julie   Kapur   8826234556  julie@yahoo.com     F   Ops     2500
Ana     Khanna  9856422312  anak@hotmail.com    F   Ops     2700
Hari    Singh   8827255666  hari@yahoo.com      M   Ops     2350
John    Kapur   9911556789  john@gmail.com      M   hr      2200
Billy   Chabra  9911664321  bily@yahoo.com      M   lgs     1900
Ginny   Singh   9857123466  ginny@yahoo.com     F   hr      2250
Emily   Kaur    8826175812  emily@gmail.com     F   Ops     2100
Amy     Sharma  9857536898  amys@hotmail.com    F   Ops     2500

We can specify ranges of characters in abbreviated form by using a hyphen. The character immediately to the left of the hyphen defines the beginning of the range and the character immediately to the right defines the end. Thus, the preceding example can be rewritten using a hyphen as follows:

$ awk '/^[ A-J ]/{ print }' emp.dat

The output on execution of the preceding code is as follows:

Jack    Singh   9857532312  jack@gmail.com      M   hr      2000
Jane    Kaur    9837432312  jane@gmail.com      F   hr      1800
Eva     Chabra  8827232115  eva@gmail.com       F   lgs     2100
Amit    Sharma  9911887766  amit@yahoo.com      M   lgs     2350
Julie   Kapur   8826234556  julie@yahoo.com     F   Ops     2500
Ana     Khanna  9856422312  anak@hotmail.com    F   Ops     2700
Hari    Singh   8827255666  hari@yahoo.com      M   Ops     2350
John    Kapur   9911556789  john@gmail.com      M   hr      2200
Billy   Chabra  9911664321  bily@yahoo.com      M   lgs     1900

Ginny   Singh   9857123466  ginny@yahoo.com     F   hr      2250
Emily   Kaur    8826175812  emily@gmail.com     F   Ops     2100
Amy     Sharma  9857536898  amys@hotmail.com    F   Ops     2500

We can print the info of employees whose names begin with 'Ja' and are followed by any two characters, as follows:

$ awk '/Ja[a-z][a-z]/{ print }' emp.dat

The output on execution of this code is as follows:

Jack    Singh   9857532312  jack@gmail.com      M   hr      2000
Jane    Kaur    9837432312  jane@gmail.com      F   hr      1800

Similarly, you can print the info of employees whose salary is either 2300 or 2500, as follows:

$ awk '/2[35]00/{ print }' emp.dat

The output on execution of the preceding code is as follows:

Julie   Kapur   8826234556  julie@yahoo.com     F   Ops     2500
Victor  Sharma  8826567898  vics@hotmail.com    M   Ops     2500
Sam     khanna  8856345512  sam@hotmail.com     F   lgs     2300
Amy     Sharma  9857536898  amys@hotmail.com    F   Ops     2500
Vina    Singh   8811776612  vina@yahoo.com      F   lgs     2300

Without both left- and right-range characters, a hyphen in a character class denotes itself, hence the [-] character classes match -:

$ echo -e "-\n
+\n
a\n
b"  | awk '/[-]/'

The output on execution of this code is as follows:

-

We can also put a hyphen at the beginning or end of a range-specified character class to match the hyphen itself, as shown here:

$  echo -e "-\n
+\n
a\n
b"  | awk '/[a-z-]/'

This can also be performed as follows:

$ echo -e "-\n
+\n
a\n
b" | awk '/[-a-z ]/'

The output on execution of the preceding code is as follows:

-
a
b

The only metacharacters valid inside the bracket expression are '\', ']', '-', or '^'. We have to put a '\' in front of them to use them inside character classes.

As in the case of anchors, if they are not placed at the appropriate position in regular expressions they lose their meaning, the same is true for the hyphen, '-', and also ']'. For example:

$ echo -e "-\n
+\n
a\n
b\n
]\n
\\" | awk '/[\^ab\-\]\\]/'

The output on execution of this code is as follows:

-
a
b
]
\

A summary of the character classes is as follows:

Pattern	Matches
[f-k]	Matches any single character between [fghijk]
[0-9]	Matches any single digit between [0123456789]
[-]	Matches a hyphen
[0-9-]	Matches any number or a hyphen
[-0-9]	Matches any number or a hyphen
[]0-9]	Matches any number or a ]
[0-9]]	Matches any number followed by ]
[0-9\-\]]	Matches any number, a hyphen, or ]
[0-9\\]	Matches any number or backslash, "\"
[\^0-9]	Matches any number or caret, "^"
[a-z]	Matches any small letter
[A-Z]	Matches any capital letter
[a-zA-Z]	Matches any alphabet
[a-zA-Z0-9]	Matches any alphanumeric character
[5-9G-Lr-z]	Matches any single character among [56789GHIJKLrstuvwxyz]
[a-zA-Z][0-9]	Matches a letter followed by a digit
[a-zA-Z-]+	Matches a letter that includes a hyphen