When we use the == condition, AWK looks for an exact match. However, when we use the match operator (~), AWK looks for a partial match. Here, ~ means contains. To match a specific pattern using regular expression, ~ and !~ are used. Regular expression comparisons are performed using a matching expression built with either of these two operators. The right-hand side of the ~ or !~ operator could be a regular expression or string enclosed between forward slashes (/…/).

Match operator (~):

It is represented as a tilde (~) symbol. It matches a pattern in a specific field. Its syntax is as follows:

expression ~ /regexpr/

It matches if the string value of the expression contains a sub-string matched by regular expression regexpr.

For example, if you want to print the records containing Singh or Kapur in last name field from the employees database emp.dat, use the ~ operator as follows:

$ awk '$2 ~ /(Singh|Kapur)/ { print }' emp.dat

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

Jack    Singh   9857532312  [email protected]      M   hr      2000
Julie   Kapur   8826234556  [email protected]     F   Ops     2500
Hari    Singh   8827255666  [email protected]      M   Ops     2350
John    Kapur   9911556789  [email protected]      M   hr      2200
Ginny   Singh   9857123466  [email protected]     F   hr      2250
Vina    Singh   8811776612  [email protected]      F   lgs     2300 

In the following example, we print the total number of users who have bash as their default login shell. In this AWK command, if the last field of a line contains the pattern bash, the AWK variable n gets incremented by one:

$ awk -F ':' '$NF ~ /bash/{n++};END { print n}' /etc/passwd

The output on execution of the preceding code is:

2 

Not match operator (!~):

It is represented by !~. It is the opposite of the ~ operator. It matches the fields that do not contain the specified pattern. Its syntax is as follows:

expression !~ /regexpr/

It matches if the string value of the expression does not contain a sub-string matched by regular expression regexpr.

For example, if you want to print the records not containing Singh or Kapur in the last name field from the employees database emp.dat, use !~ as follows:

$ awk '$2 !~ /(Singh|Kapur)/ { print }' emp.dat

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

Jane    Kaur    9837432312  [email protected]      F   hr      1800
Eva     Chabra  8827232115  [email protected]       F   lgs     2100
Amit    Sharma  9911887766  [email protected]      M   lgs     2350
Ana     Khanna  9856422312  [email protected]    F   Ops     2700
Victor  Sharma  8826567898  [email protected]    M   Ops     2500
Billy   Chabra  9911664321  [email protected]      M   lgs     1900
Sam     khanna  8856345512  [email protected]     F   lgs     2300
Emily   Kaur    8826175812  [email protected]     F   Ops     2100
Amy     Sharma  9857536898  [email protected]    F   Ops     2500

Regular expression operators: