Regular Expressions

Use regular expressions

/regexp/

awk '/li/ { print $2 }' mail-list

exp ~ /regexp/

awk '$1 ~ /^J/' inventory-shipped

exp !~ /regexp/

awk '$1 !~ /^J/' inventory-shipped

echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'

Dynamic regexps

ls -l | awk 'BEGIN { digits_regexp = "[[:digit:]]+" } $5 ~ digits_regexp { print }'
ls -l | awk 'BEGIN { digits_regexp = "[[:alpha:]]+" } $4 ~ digits_regexp { print }'

POSIX character classes

Class	Description
[: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 1
[:lower:]	Lowercase alphabetic characters
[:print:]	Printable characters 2
[:punct:]	Punctuation characters 3
[:space:]	Space characters 4
[:upper:]	Uppercase alphabetic characters
[:xdigit:]	Characters that are hexadecimal digits

Ignore case

IGNORECASE = [0,1]

echo -e "ab\ncd" | awk 'BEGIN { IGNORECASE = 1 } /A/ { print }'

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Footnote

1

A space is printable but not visible, whereas an ‘ a ’ is both.

2

Characters that are not control characters.

3

Characters that are not letters, digits, control characters, or space characters.

4

Such as space, TAB, and formfeed, to name a few.