Regular expressions as string-matching patterns with AWK

Regular expressions are used as string-matching patterns with AWK in the following three ways. We use the '~' and '! ~' match operators to perform regular expression comparisons:

• /regexpr/: This matches when the current input line contains a sub-string matched by regexpr. It is the most basic regular expression, which matches itself as a string or sub-string. For example, /mail/ matches only when the current input line contains the mail string as a string, a sub-string, or both. So, we will get lines with Gmail as well as Hotmail in the email ID field of the employee database as follows:

$ awk '/mail/' 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
Eva     Chabra  8827232115  eva@gmail.com       F   lgs     2100
Ana     Khanna  9856422312  anak@hotmail.com    F   Ops     2700
Victor  Sharma  8826567898  vics@hotmail.com    M   Ops     2500
John    Kapur   9911556789  john@gmail.com      M   hr      2200
Sam     khanna  8856345512  sam@hotmail.com     F   lgs     2300
Emily   Kaur    8826175812  emily@gmail.com     F   Ops     2100
Amy     Sharma  9857536898  amys@hotmail.com    F   Ops     2500

In this example, we do not specify any expression, hence it automatically matches a whole line, as follows:

$ awk '$0 ~ /mail/' 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
Eva     Chabra  8827232115  eva@gmail.com       F   lgs     2100
Ana     Khanna  9856422312  anak@hotmail.com    F   Ops     2700
Victor  Sharma  8826567898  vics@hotmail.com    M   Ops     2500
John    Kapur   9911556789  john@gmail.com      M   hr      2200
Sam     khanna  8856345512  sam@hotmail.com     F   lgs     2300
Emily   Kaur    8826175812  emily@gmail.com     F   Ops     2100
Amy     Sharma  9857536898  amys@hotmail.com    F   Ops     2500

• expression ~ /regexpr /: This matches if the string value of the expression contains a sub-string matched by regexpr. Generally, this left-hand operand of the matching operator is a field. For example, in the following command, we print all the lines in which the value in the second field contains a /Singh/ string:

$ awk '$2 ~ /Singh/{ print }' emp.dat

We can also use the expression as follows:

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

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

Jack    Singh   9857532312  jack@gmail.com      M   hr      2000
Hari    Singh   8827255666  hari@yahoo.com      M   Ops     2350
Ginny   Singh   9857123466  ginny@yahoo.com     F   hr      2250
Vina    Singh   8811776612  vina@yahoo.com      F   lgs     2300

• expression !~ /regexpr /: This matches if the string value of the expression does not contain a sub-string matched by regexpr. Generally, this expression is also a field variable. For example, in the following example, we print all the lines that don't contain the Singh sub-string in the second field, as follows:

$ awk '$2 !~ /Singh/{ print }' emp.dat

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

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
Victor  Sharma  8826567898  vics@hotmail.com    M   Ops     2500
John    Kapur   9911556789  john@gmail.com      M   hr      2200
Billy   Chabra  9911664321  bily@yahoo.com      M   lgs     1900
Sam     khanna  8856345512  sam@hotmail.com     F   lgs     2300
Emily   Kaur    8826175812  emily@gmail.com     F   Ops     2100
Amy     Sharma  9857536898  amys@hotmail.com    F   Ops     2500

Any expression may be used in place of /regexpr/ in the context of ~; and !~. The expression here could also be if, while, for, and do statements.