Take an address fragment only where the whole line is in hand

:BTags reads the address field on its own, so a tab inside a ctags pattern
leaves a fragment such as '/^', which matched whatever line came next and
recorded it in the quickfix list. ctags always terminates its patterns, so on
the field alone a fragment is the cut and is refused. The '/*:Ag*' of a helptags
file has no terminator and still goes through, :Tags having the whole line.

autoload/fzf/vim.vim

@@ -547,21 +547,21 @@ function! s:sandboxed(cmd)
   return 'sandbox execute '.string(a:cmd)
 endfunction
 
-" Address of a tag, given the rest of the line after the file name. Only the
+" Address of a tag, given the rest of the line after the file name, or, with
-" forms a tags file is meant to hold, a line number and a search pattern,
+" 'whole' off, the address field alone as :BTags reads it. Only the forms a tags
-" chained with ';' for '--excmd=combine' and cut at the ';"' terminator as
+" file is meant to hold, a line number and a search pattern, chained with ';' for
-" Vim's find_extra() does. Returns an empty string for anything else, which is
+" '--excmd=combine' and cut at the ';"' terminator as Vim's find_extra() does.
-" then not run.
+" Returns an empty string for anything else, which is then not run.
 "
-" A pattern holding a tab reaches :BTags cut short, since it reads the field on
+" A pattern holding a tab reaches :BTags cut short. Where earlier parts parsed,
-" its own. Where earlier parts parsed, keep those and drop the fragment, since a
+" keep those, since a line number beats half a pattern. A fragment on its own is
-" line number beats half a pattern. A fragment on its own is indistinguishable
+" an address only where the whole line is in hand, as in a helptags file, whose
-" from a helptags address such as '/*:Ag*', so it goes through unterminated, the
+" '/*:Ag*' carries no terminator. ctags always terminates, so on the field alone
-" way a builtin tag jump takes the same field. Running to the end of the line, it
+" a fragment is the cut, and it would match some other line. Running to the end
-" takes any '|' or ';' with it, so it cannot chain a command either.
+" of the line it takes any '|' or ';' with it, so it cannot chain a command.
 let s:tag_address_part = '\%(\d\+\|/\%(\\.\|[^/\\]\)*/\|?\%(\\.\|[^?\\]\)*?\)'
 let s:tag_address_open = '\%(/\%(\\.\|[^/\\]\)*\|?\%(\\.\|[^?\\]\)*\)'
-function! s:tag_address(rest)
+function! s:tag_address(rest, whole)
   " :BTags aligns its columns, so its field arrives padded
   let rest = s:strip(a:rest)
   let chain = '^'.s:tag_address_part.'\%(;'.s:tag_address_part.'\)*'
@@ -570,7 +570,10 @@ function! s:tag_address(rest)
     return address
   endif
   let address = matchstr(rest, chain.'\ze;[/?]')
-  return empty(address) ? matchstr(rest, '^'.s:tag_address_open.'$') : address
+  if !empty(address)
+    return address
+  endif
+  return a:whole ? matchstr(rest, '^'.s:tag_address_open.'$') : ''
 endfunction
 
 " [key, [filename, [stay_on_edit: 0]]]
@@ -768,7 +771,7 @@ function! s:goto_entry(winid, bufnr, dir, kind, entry) abort
     if len(parts) < 3
       return
     endif
-    let excmd = s:tag_address(join(parts[2:-2], '')[:-2])
+    let excmd = s:tag_address(join(parts[2:-2], '')[:-2], 1)
     let relpath = parts[1][:-2]
     let path = relpath =~ (s:is_win ? '^[A-Z]:\' : '^/')
           \ ? relpath : join([fnamemodify(parts[-1], ':h'), relpath], '/')
@@ -783,7 +786,7 @@ function! s:goto_entry(winid, bufnr, dir, kind, entry) abort
     if len(parts) < 3 || !bufexists(a:bufnr)
       return
     endif
-    let excmd = s:tag_address(parts[2])
+    let excmd = s:tag_address(parts[2], 0)
     if empty(excmd)
       return
     endif
@@ -1703,7 +1706,7 @@ function! s:btags_sink(from, lines)
     for line in a:lines[1:]
       try
         let parts = split(line, "\t")
-        let excmd = s:tag_address(parts[2])
+        let excmd = s:tag_address(parts[2], 0)
         if empty(excmd)
           throw 'Unsupported tag address: '.s:strip(parts[0])
         endif
@@ -1786,7 +1789,7 @@ function! s:tags_sink(from, lines)
     for line in list
       try
         let parts   = split(line, '\t\zs')
-        let excmd   = s:tag_address(join(parts[2:-2], '')[:-2])
+        let excmd   = s:tag_address(join(parts[2:-2], '')[:-2], 1)
         if empty(excmd)
           throw 'Unsupported tag address: '.s:strip(parts[0])
         endif