Use regular expression at filetype detection

ftdetect/requirements.vim

@@ -1,4 +1,17 @@
-au BufNewFile,BufRead *requirements.{txt,in} set ft=requirements
+function! s:isRequirementsFile()
-au BufNewFile,BufRead *requirements/*.{txt,in} set ft=requirements
+    let l:filename = expand("%:p")
+
+    if l:filename =~# '\v.*requirements\.(txt|in)'
+        return 1
+    endif
+
+    if l:filename =~# '\v.requirements/.*\.(txt|in)'
+        return 1
+    endif
+
+    return 0
+endfunction
+
+au BufNewFile,BufRead *.{txt,in} if s:isRequirementsFile() | set ft=requirements
 
 " vim: et sw=4 ts=4 sts=4: