<nop> instead of an exclamation mark, but I believe <nop> is now deprecated and no longer documented in TextFormattingRules.
---+++ !WikiWord at Start of Heading
---+++ Another test with !WikiWord in the middle
---+++ checking acronyms like !AJAX
---+++ and <nop>NopTag still works
---+++ not escaped WikiWord
---+++ not escaped AJAX
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