Regex that matches comma separated list of 4 strings in bash-CodePudding

Im trying to write a regex function in bash where the first and third thing in the list are the same and the second and fourth thing in the list are the same.

grep -E "^(([0-9a-zA-Z][0-9a-zA-Z_]*)([,][0-9a-zA-Z][0-9a-zA-Z_]*)*)$"

This is my regular expression as of now. I am unsure on how to check if the first and third, second and fourth are the same. Say for example a,b,a,b matches but a,b,b,b doesnt.

CodePudding user response：

It is quite straightforward with awk:

awk -F, '$1 == $3 && $2 == $4'

CodePudding user response：

Use a back-reference to check that one part matches a previously captured string.

grep -E -i "^([0-9a-z][0-9a-z_]*),([0-9a-z][0-9a-z_]*),\1,\2$"

CodePudding user response：

With GNU sed:

echo 'a,b,a,b' | sed -E '/(.*),(.*),\1,\2/!d'

Or only which bash:

x='a,b,a,b'
[[ $x =~ (.*),(.*),(.*),(.*) ]] &&
[[ ${BASH_REMATCH[1]} == ${BASH_REMATCH[3]} &&
   ${BASH_REMATCH[2]} == ${BASH_REMATCH[4]} ]] &&
echo "okay"

or without regex:

IFS=, read -r a b c d <<<"a,b,a,b"; [[ $a == $c && $b == $d ]] && echo "okay"

Output:

a,b,a,b