custom rule with pattern test of 0 (zero) not processed

[dms dms]

Hello all,
  I searched high and low but could not find anywhere that says the
following rule patten is invalid or not allowed in SpamAssassin 3.2.5 on
CentOS 5.1  However it works with non-zero numbers... Thoughts? and TIA

header          LOCAL_TESTHEADER        TESTHEADER =~ /^0$/
score           LOCAL_TESTHEADER        1000



message:
TESTHEADER: 0
From:[EMAIL PROTECTED] <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Subject: This is a TESTHEADER: 0 test

blahblah blah