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awkgnugawkcharacter-class

Regex "^[[:digit:]]$" not working as expected in AWK/GAWK

My GAWK version on RHEL is:

gawk-3.1.5-15.el5

I wanted to print a line if the first field of it has all digits (no special characters, even space to be considered)

Example:

echo "123456789012345,3" | awk -F, '{if ($1 ~ /^[[:digit:]]$/)  print $0}'

Output:
Nothing

Expected Output:
123456789012345,3

What is going wrong here ? Does my AWK version not understand the GNU character classes ? Kindly help

Solution

  • To match multiple digits in the the [[:digit:]] character class add a +, which means match one or more number of digits in $1.

    echo "123456789012345,3" | awk -F, '{if ($1 ~ /^([[:digit:]]+)$/)  print $0}'
123456789012345,3

    which satisfies your requirement.

    A more idiomatic way ( as suggested from the comments) would be to drop the print and involve the direct match on the line and print it,

    echo "123456789012345,3" | awk -F, '$1 ~ /^([[:digit:]]+)$/'
123456789012345,3

    Some more examples which demonstrate the same,

    echo "a1,3" | awk -F, '$1 ~ /^([[:digit:]]+)$/'

    (and)

    echo "aa,3" | awk -F, '$1 ~ /^([[:digit:]]+)$/'

    do NOT produce any output a per the requirement.

    Another POSIX compliant way to do strict length checking of digits can be achieved with something like below, where {3} denotes the match length.

    echo "123,3" |  awk --posix -F, '$1 ~ /^[0-9]{3}$/'
123,3

    (and)

    echo "12,3" |  awk --posix -F, '$1 ~ /^[0-9]{3}$/'

    does not produce any output.

    If you are using a relatively newer version of bash shell, it supports a native regEx operator with the ~ using POSIX character classes as above, something like

    #!/bin/bash

while IFS=',' read -r row1 row2
do
   [[ $row1 =~ ^([[:digit:]]+)$ ]] && printf "%s,%s\n" "$row1" "$row2"
done < file

    For an input file say file

    $ cat file
122,12
a1,22
aa,12

    The script produces, 

    $ bash script.sh
122,12

    Although this works, bash regEx can be slower a relatively straight-forward way using string manipulation would be something like

    while IFS=',' read -r row1 row2
do
   [[ -z "${row1//[0-9]/}" ]] && printf "%s,%s\n" "$row1" "$row2"
done < file

    The "${row1//[0-9]/}" strips all the digits from the row and the condition becomes true only if there are no other characters left in the variable.