search: common Subject: normalization for Re: prefixes

Drop German ("Aw:") support since it's non-standard and
is not supported by Mail::Thread and non-English prefixes
are more likely to conflict with prefixes used in Free Software
development where ("subsection:") prefixes are common and English is the
common language.

Anyways we don't filter "Vs: " (Finnish) or "Sv: "
(Norwegian, Swedish, Danish, Icelandic), either.

ref:
https://en.wikipedia.org/wiki/RE_(e-mail)#Abbreviations_in_other_languages

1 files changed, 5 insertions, 1 deletions

diff --git a/lib/PublicInbox/Search.pm b/lib/PublicInbox/Search.pm
index db86301d..6a05ce7a 100644
--- a/lib/PublicInbox/Search.pm
+++ b/lib/PublicInbox/Search.pm
@@ -10,6 +10,10 @@ require PublicInbox::View;
 use Email::MIME;
 use PublicInbox::MID qw/mid_clean mid_compressed/;
 
+# This is English-only, everything else is non-standard and may be confused as
+# a prefix common in patch emails
+our $REPLY_RE = qr/^re:\s+/i;
+
 use constant {
         TS => 0,
         # SCHEMA_VERSION history
@@ -490,7 +494,7 @@ sub subject_path {
 
         $subj =~ s/\A\s+//;
         $subj =~ s/\s+\z//;
-        $subj =~ s/^(?:re|aw):\s*//i; # remove reply prefix (aw: German)
+        $subj =~ s/$REPLY_RE//igo; # remove reply prefix
         $subj =~ s![^a-zA-Z0-9_\.~/\-]+!_!g;
         lc($subj);
 }