Question

I can run a while loop with regex successfully

$ cat while.sh
#!/bin/sh
arr=(a1c a2c a3c b4c)
i=0
while [[ ${arr[i]} =~ a(.)c ]]
do
  echo ${BASH_REMATCH[1]}
  ((i++))
done

$ ./while.sh
1
2
3

A for loop causes this error

$ cat for.sh
#!/bin/sh
arr=(a1c a2c a3c b4c)
for ((i=0; [[ ${arr[i]} =~ a(.)c ]]; i++))
do
  echo ${BASH_REMATCH[1]}
done

$ ./for.sh
./for.sh: line 3: ((: [[ a1c =~ a(.)c ]]: syntax error: operand expected (error
token is "[[ a1c =~ a(.)c ]]")

Answer 1

I’m not sure your for loop construct is legal with regex. Double parenthesis are for Arithmetic expressions and that includes for loops. Regex matching is not arithmetic. I think if you were really set on using for for some reason, you would have to do something like:

arr=(a1c a2c a3c b4c)

for val in "${arr[@]}"; do
    if [[ $val =~ a(.)c ]]; then
        echo ${BASH_REMATCH[1]}
    fi
done