faq: on searching punctuation and regexp

diff --git a/faq.mdwn b/faq.mdwn
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--- a/faq.mdwn
+++ b/faq.mdwn
@@ -41,3 +41,15 @@ Next, use that to query the messages:
 
         $ notmuch search $(notmuch search --output=tags \* | \
                 sed 's/^/not tag:/;2~1s/^/and /')
+
+## How do I search for punctuation, specific special characters, or regexp?
+
+Please see the [[notmuch-search-terms manual
+page|manpages/notmuch-search-terms-7]] first,
+
+The main thing to understand is that Xapian, and therefore Notmuch, searches are
+closer to natural language searches than regular expression
+searches. Punctuation is mostly ignored.
+
+The boolean prefix searches (see Boolean and Probabilistic Prefixes in the man
+page), such as tag: or path: searches, need an exact match.