Use regex with for loop? - regex

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

To follow-up on my comment above, if you wanted to keep your formatting, more or less, this might do what you expected, because you need the return value of the evaluation as an expression in the for loop, not the output of it.
for ((i=0; `[[ ${arr[i]} =~ a(.)c ]] && echo -n 1`; i++)); do
# do whatever
done
Ugly, but worked for me, and should explain your error for you. The back-ticks evaluate the expression then output either a '1' for true or nothing for false. This leaves you the valid conditional for the loop in the middle.

Answer 2

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