AWK: Get/Extract String Matching Regexp

Example of how to use AWK in order to get/extract/return parts of lines/strings, matching a "dynamic" regular expression (specified through a shell environment variable).

Source file used in this example

$ cat example.txt

Attr[27716, Field[4937190,usage=51]]

Attr[27716, Field[4937191,usage=10]]

Attr[27716, Field[4937192,usage=321]]

Two usages

# Defining the regular expression pattern as environment variable

$ MY_PATTERN="([0-9]+),used"

# Using the predefined awk variables RSTART and RLENGTH

# Returns the whole string matching the regexp

$ awk -v PATTERN="${MY_PATTERN}" 'match($0, PATTERN) { print substr( $0, RSTART, RLENGTH) }' example.txt

4937190,usage

4937191,usage

4937192,usage

# Using the capture group & array functionality in order to get only the regexp part in parenthesis

$ awk -v PATTERN="${MY_PATTERN}" 'match($0, PATTERN, array) { print array[1] }' example.txt

4937190

4937191

4937192