GNU AWK - Part 5

Regular Expressions in AWK

The syntax for using regular expressions to match lines in AWK is as follows:

word ~ /match/

Or for not matching, use the following:

word !~ /match/

Remember the following file from the previous episodes:

name color amount

apple red 4

banana yellow 6

strawberry red 3

grape purple 10

apple green 8

plum purple 2

kiwi brown 4

potato brown 9

pineapple yellow 5

We can run the following command:

$1 ~ /p[elu]/ {print $0}

We will get the following output:

apple red 4

grape purple 10

apple green 8

plum purple 2

pineapple yellow 5

In another example:

$2 ~ /e{2}/ {print $0}

Will produce the output:

apple green 8

Regular expression basics

Certain characters have special meaning when using regular expressions.

Anchors

^ - beginning of the line

$ - end of the line

A - beginning of a string

z - end of a string

b on a word boundary

Characters

[ad] - a or d

[a-d] - any character a through d

[^a-d] - not any character a through d

w - any word

s - any white-space character

d - any digit

The capital version of w, s, and d are negations.

Or, you can reference characters the POSIX standard way:

[:alnum:] - Alphanumeric characters

[:alpha:] - Alphabetic characters

[:blank:] - Space and TAB characters

[:cntrl:] - Control characters

[:digit:] - Numeric characters

[:graph:] - Characters that are both printable and visible (a space is printable but not visible, whereas an ‘a’ is both)

[:lower:] - Lowercase alphabetic characters

[:print:] - Printable characters (characters that are not control characters)

[:punct:] - Punctuation characters (characters that are not letters, digits, control characters, or space characters)

[:space:] - Space characters (such as space, TAB, and formfeed, to name a few)

[:upper:] - Uppercase alphabetic characters

[:xdigit:] - Characters that are hexadecimal digits

Quantifiers

. - match any character

+ - match preceding one or more times

* - match preceding zero or more times

? - match preceding zero or one time

{n} - match preceding exactly n times

{n,} - match preceding n or more times

{n,m} - match preceding between n and m times

Grouped Matches

(...) - Parentheses are used for grouping

| - Means or in the context of a grouped match

Replacement

The sub command substitutes the match with the replacement string. This only applies to the first match.

The gsub command substitutes all matching items.

The gensub command command substitutes the in a similar way as sub and gsub, but with extra functionality

The