@misc: Improve formatting of arXiv preprints

May be considered as `acl_natbib_nourl.bst` for urlbst processing

Works with `@misc` type only.
Supports `archivePrefix` or `eprinttype`.
Sets `arXiv preprint` if `publisher = {arXiv}` and no `eprint` number is provided.
This will work with arXiv generated bibs (more info at https://tex.stackexchange.com/a/679743) and some doi2bib.org bibs generated from arxiv doi.

No support for `primaryClass` (no example from ACL), no detection of versions from `url` (seems too hard to me), and no support for `@article` with `eprint` from DBLP (too much to test for now — may get worse).

P.S. This addition was originally inspired by `output.eprint` from `ACM-Reference-Format.bst` from acmart, but later diverged.

latex/acl_natbib.bst

@@ -53,12 +53,15 @@
 
 ENTRY
   { address
+    archivePrefix
     author
     booktitle
     chapter
     edition
     editor
     eid
+    eprint
+    eprinttype % = archivePrefix
     howpublished
     institution
     journal
@@ -893,6 +896,45 @@ FUNCTION {format.organization.address}
 { organization "organization" bibinfo.check format.org.or.pub
 }
 
+FUNCTION {archiveprefix.or.eprinttype} % holder for eprinttype with archiveprefix precedence
+{
+  archiveprefix empty$
+  {
+    eprinttype empty$
+      { "" } % not using 'skip$ to reduce errors like "nothing to pop from stack"
+      { eprinttype }
+    if$
+  }
+  { archiveprefix }
+  if$
+}
+
+FUNCTION {output.eprint} % this is only used with the @misc record type (common for arXiv and other preprint server bibtex records)
+{
+  eprint empty$
+    {% if eprint field is empty
+      publisher field.or.null "arXiv" = % field.or.null here helps when no publisher field in the record
+        { publisher " preprint" * } % add " preprint" to publisher with the idea that publisher is the name of the preprint server
+        { "" } % if publisher != "arXiv" then empty output
+      if$
+      emphasize % no output function after emphasize because nothing goes after this
+    }
+    {% if eprint field is not empty
+      archiveprefix.or.eprinttype empty$
+        { "" } % not using 'skip$ to reduce errors like "nothing to pop from stack"
+        {% if archiveprefix or eprinttype fields are not empty
+          journal empty$
+            { "Preprint" } % if journal field is empty: output just "Preprint" emphasized like a journal name
+            { journal } % if journal field is not empty, output it (takes precedence)
+          if$
+          emphasize output % emphasize what we formed before, setting output as a border to the subblock that follows with the comma delimiter
+          archiveprefix.or.eprinttype ":" * eprint * % subblock with eprinttype and eprint number
+        }
+      if$
+    }
+  if$
+}
+
 FUNCTION {article}
 { output.bibitem
   format.authors "author" output.check
@@ -1098,6 +1140,8 @@ FUNCTION {misc}
   new.block
   howpublished "howpublished" bibinfo.check output
   new.block
+  output.eprint output
+  new.block
   format.note output
   fin.entry
 }