The "X-Label" patch provides two new expanders for the $index_format configuration variable, and a new pattern-matching selector. %y and %Y expand (in slightly different ways) to the value of a message's X-Label: header field, and ~y performs regular-expression matching on the value of a message's X-Label: field. The following texts are from messages posted to the Mutt developers' mailing list:

Message 1: From: David Champion Subject: Three patches: kerberos, keymap_defs.h, hdr_format To: The Mutt Developers List Date: Thu, 23 Mar 2000 04:10:58 -0600 Message-ID: <20000323041058.B20382@smack.uchicago.edu> ... fluff, but it's useful to me. The patch makes parse.c:mutt_read_rfc822_header() cache any X-Label: header present in a message's header block. %y then expands to the value of that header. I use this for marking mailing lists. (The "subscribe" and "lists" directives are not really what I want; I want procmail to be able to mark message lists or categories however I choose, and mutt to label messages accordingly.) I use this hdr_format string: "%Z %4C %[%y%m%d] %-18.18F [%4c] %?y?[%y] ?%s%| " This makes the X-Label label show in brackets iff it is present, and the message is not a descendent in a threaded tree diagram. (If it is, or the label is absent, "%?y?[%y] ?" expands to nothing.)

Message 2: From: David Champion Subject: Re: X-Label and %y To: The Mutt Developers List References: <20000323041058.B20382@smack.uchicago.edu> <20000323041058.B20382@smack.uchicago.edu> <20000323115532.A20372@sobolev.rhein.de> <20000323195733.D10666@chamber.dna.fi> <20000324012154.C28736@faveve.uni-stuttgart.de> <20000323194020.A4437@smack.uchicago.edu> <20000324084827.E28519@sobolev.rhein.de> ... + %y expands to the X-Label's value, no matter what. + %?y tests true if X-Label is present — if %y would have value. + %Y expands to the X-Label if X-Label exists, and any of these is true: 1. this message is unthreaded 2. this message is at the top of its thread 3. a. this message is its parent's first child, and its parent has no X-Label 3. b. this message is its parent's first child, and its parent's X-Label is different 4. a. this message is not its parent's first child, and its previous sibling has no X-Label 4. b. this message is not its parent's first child, and its previous sibling's X-Label is different + %?Y tests true if %Y would have value + ~y just regex-matches the X-Label's value As Aaron Schwab noted, it's hard to tell a %Y with no X-Label from a %Y whose predecessor's X-Label is the same. All I can say is: "%y?%Y&none?". Here are some examples I threw together. I wasn't sure where in the docs they might fit. These are all variations on the default $index_format. ## If X-Label is present, show it in [brackets], regardless of tree view. ## Warning! This will mess up tree diagramming characters! #set index_format="%4C %Z %{%b %d} %-15.15L (%4l) [%y] %s" ## Same thing, but always use up exactly 8 columns for the label #set index_format="%4C %Z %{%b %d} %-15.15L (%4l) [%-8.8y] %s" ## Same thing, but fill the gap with dashes if no X-Label is present #set index_format="%4C %Z %{%b %d} %-15.15L (%4l) [%?y?%-8.8y&--------?] %s" ## If X-Label is present, show any X-Label that is at the top of a tree, ## or which is different from its predecessor's X-Label. Use a ## fixed-width field to preserve sanity of tree diagrams. #set index_format="%4C %Z %{%b %d} %-15.15L (%4l) %?y?[%-5.5Y] ?%s" ## If X-Label is present and different from its predecessor's X-Label, ## show its first 5 characters, left-justified in a 5-column field, ## enclosed in [brackets]. Otherwise, fill an equivalent span with ## whitespace. #set index_format="%4C %Z %{%b %d} %-15.15L (%4l) %?Y?[%-5.5Y] & %-5.0Z ?%s" ## Same as previous, but do not respect tree diagrams at all. Show ## X-Labels that are different from their predecessors in the tree ## view, or which are not tree members, and do not consume any space ## otherwise. This is nice when the X-labels are not expected to ## vary within a tree (e.g., for mailing lists). #set index_format="%4C %Z %{%b %d} %-15.15L (%4l) %?Y?[%Y] ?%s"