get header in variable and use it to match rule

I have a procmail rule that works perfectly for me:

:0
* ^To:.*anonymized
* ^Subject:.*Report domain: anonymized Submitter: [^ ]* Report-ID:
{
LOG="1"
:0 cw
* ^To:\/.+
| /etc/procmail/process_dmarc_report.sh "$MATCH"
}

But the subject is sometimes encoded, which breaks the rule. Based on https://stackoverflow.com/a/29715987/13463349

I wanted to replace the test on the Subject header with a test on the SUBJECT variable containing the possibly decoded subject, as follows:

:0
* ^Subject:.*=\?utf-8\?B\?
SUBJECT=`formail -cXSubject: | perl -MMIME::Base64 -ne 's/^=\?utf-8\?B\?(.*?)\?=$/print decode_base64($1)/e'`
:0 E
SUBJECT=`formail -cXSubject:`
:0
* ^To:.*anonymized
* SUBJECT ?? Subject:.*Report domain: anonymized Submitter: [^ ]* Report-ID:.*

The subject is correctly decoded, but I'm doing it wrong because I have an error in the logs:

procmail: Skipped "[Preview] Report Domain: anonymized Submitter: enterprise.protection.outlook.com Report-ID: c136884d7adc4f5c82e2fdbbde75e2ab"

And the email is delivered to Folder: SUBJECT=Subject:.

I tried with -x instead of -X and not including Subject: in the test, and several other small adaptations, but without success. I can't find my mistake. It may be simple, but I can't seem to get it right.

If, as suggested, I do something like:

:0
* ^Subject:.*=\?utf-8\?B\?
SUBJECT=| formail -cXSubject: | perl -MMIME::Base64 -ne 's/^=\?utf-8\?B\?(.*?)\?=$/print decode_base64($1)/e'

:0 E
SUBJECT=| formail -cXSubject:

$SUBJECT is empty

My entire rule (not working) at the moment:

:0
* ^Subject:.*=\?utf-8\?B\?
SUBJECT=`formail -cXSubject: | perl -MMIME::Base64 -ne 's/^=\?utf-8\?B\?(.*?)\?=$/print decode_base64($1)/e'`
:0 E
SUBJECT=`formail -cXSubject:`
:0
* ^To:.*anonymized
* SUBJECT ?? Subject:.*Report domain: anonymized Submitter: [^ ]* Report-ID:.*
{
        LOG="1"
        :0 cw
        * ^To:\/.+
        | /etc/procmail/process_dmarc_report.sh "$MATCH"
}

Solution

The problem was about the variable test:

* ^Subject:.*Report domain: anonymized Submitter: [^ ]* Report-ID:

won't work because of [^ ]*
but

* ^Subject:.*Report domain: anonymized Submitter: .* Report-ID:

works !

* variable ?? regex
Test the value of variable against regex.

The documentation says it's a regex, but it really isn't.

My final working rule is:

        :0
        * ^Subject: \/=\?utf-8\?B\?.+
        {
                SUBJECT=`echo $MATCH | perl -MMIME::Base64 -ne 's/^=\?utf-8\?B\?(.*?)\?=$/print decode_base64($1)/e'` 
        }

        :0E
        * ^Subject: \/.+
        {
                SUBJECT=$MATCH
        }


        :0
        * SUBJECT ?? Report domain: novazur\.fr Submitter: .* Report-ID:
        {
                :0 cw
                * ^To:\/.+
                | /etc/procmail/process_dmarc_report.sh "$MATCH"
        }

NB: Never remove curly braces that you might think are unnecessary around variable assignments. They are essential, even in:

:0
* ^Subject: \/=\?utf-8\?B\?.+
{
    SUBJECT=`echo $MATCH | perl -MMIME::Base64 -ne 's/^=\?utf-8\?B\?(.*?)\?=$/print decode_base64($1)/e'`
}
:0E
* ^Subject: \/.+
{
    SUBJECT=$MATCH
}