Flexible prefix in the forced subject regex

[SoraSuegami]

We are working to make the prefix in the forced subject regex more flexible, ensuring it remains agnostic to any prefix added in reply emails.
I attempted the following three patterns, but all of them failed to compile with the error: 'Failed to build DFA for regex: "", please check your regex. Error: error building NFA", which occurs in regex-automata library depended by the zk-regex compiler. (The in the error is the regex string of the decomposed part including "[^[]*" for each decomposed regex.)

{
    "parts": [
        {
            "is_public": false,
            "regex_def": "(\r\n|^)subject:"
        },
        {
            "is_public": false,
            "regex_def": "[^[]*"
        },
        {
            "is_public": false,
            "regex_def": "\\[Reply Needed\\][^\r\n]*\r\n"
        }
    ]
}

{
    "parts": [
        {
            "is_public": false,
            "regex_def": "(\r\n|^)subject:[^[]*"
        },
        {
            "is_public": false,
            "regex_def": "\\[Reply Needed\\][^\r\n]*\r\n"
        }
    ]
}

{
    "parts": [
        {
            "is_public": false,
            "regex_def": "(\r\n|^)subject:[^[]*\\[Reply Needed\\][^\r\n]*\r\n"
        }
    ]
}

[SoraSuegami]

When I tried "[^\[]" instead of "[^[]", I got a parse error because \ is unnecessary in the negation regex.

[Divide-By-0]

And if you double the backslash?

https://docs.prove.email/zk-email-sdk/create-blueprint quotes:

Some characters need to be escaped in the regex. For example, the " character needs to be escaped as ". This way when the JSON is parsed, it is not confused with the JSON syntax.

Also all the regex reserved characters need to be escaped twice. For example, the . character needs to be escaped as \.. One for the JSON parser and one for the regex engine.

[SoraSuegami]

Sorry for the confusion.

When I tried "[^\[]" instead of "[^[]", I got a parse error because \ is unnecessary in the negation regex.

This means I tried the double backslash in the decomposed json as a single \ was not permitted in a JSON syntax.
However, I got the error Error: Parse Error: Invalid character: \\.
I think we need to modify the zk-regex compiler.