Why does BASH_REMATCH return empty when the script file is sourced from zsh?

Question

So I created myself a cute little bash script:

#!/usr/bin/env bash

TEXT="FOO BAR"
REGEX="FOO (.+)"

if [[ $TEXT =~ $REGEX ]]; then 
    echo "Matched ${BASH_REMATCH[1]}"
else
    echo "No match."
fi

Pretty basic. If I run ./cutescript.sh , then I get the result:

Matched BAR

But if I source the script, . ./cutescript.sh . ./cutescript.sh , then it still matches, but BASH_REMATCH[1 is empty.

Wat.

I'm on MacOS Catalina 10.15.7 (19H2), executing this in zsh , but with the bash shebang.

Can anyone explain this irregularity?

Answer 1

When you read the file with . , it gets executed by whatever shell you're running - in this case, you said you're in zsh. The name of the BASH_REMATCH array is, as the name implies, specific to bash ; if your shell is zsh, for example, then the matched text in this case would be found in $match[1] instead. And I don't think ksh does true regex matching at all.

Answer 2

Now, armed with the knowledge that BASH_REMATCH doesn't exist natively in zsh , I did a little more digging:

This post is actually a duplicate. There's another question here that explains the solution: BASH_REMATCH doesn't capture

Setting options KSH_ARRAYS BASH_REMATCH allows zsh to emulate bash 's regular expression features.

A simple way to make the above script compatible with zsh is:

#!/usr/bin/env bash

# Ensures that BASH_REMATCH works if called in zsh.
setopt KSH_ARRAYS BASH_REMATCH

TEXT="FOO BAR"
REGEX="FOO (.+)"

if [[ $TEXT =~ $REGEX ]]; then 
    echo "Matched ${BASH_REMATCH[1]}"
else
    echo "No match."
fi

unsetopt KSH_ARRAYS BASH_REMATCH

Also another related question: What is the zsh equivalent for $BASH_REMATCH[]?