Unlike glibc, msys2-runtime does not seem to implement \w and \s.
This replaces them with equivalent classes.
--- a/scripts/completion/bash_completion.in
+++ b/scripts/completion/bash_completion.in
@@ -24,7 +24,7 @@
 }
 
 _arch_incomp() {
-  local r="\s-(-${1#* }\s|\w*${1% *})"; [[ $COMP_LINE =~ $r ]]
+  local r="[[:space:]]-(-${1#* }[[:space:]]|[[:alnum:]_]*${1% *})"; [[ $COMP_LINE =~ $r ]]
 }
 
 _pacman_keyids() {
@@ -74,7 +74,7 @@
   local cur opts prev
   COMPREPLY=()
   _get_comp_words_by_ref cur prev
-  if [[ $cur = -* && ! $prev =~ ^-(-(config|help|key|version)$|\w*[Vhp]) ]]; then
+  if [[ $cur = -* && ! $prev =~ ^-(-(config|help|key|version)$|[[:alnum:]_]*[Vhp]) ]]; then
     opts=('allsource asdeps check clean cleanbuild config force geninteg help
            holdver ignorearch install key log needed noarchive nobuild nocheck
            nocolor noconfirm nodeps noextract noprepare noprogressbar nosign
@@ -123,7 +123,7 @@
 
   if [[ $? != 0 ]]; then
     _arch_ptr2comp core
-  elif [[ ! $prev =~ ^-\w*[Vbhr] &&
+  elif [[ ! $prev =~ ^-[[:alnum:]_]*[Vbhr] &&
     ! $prev = --@(cachedir|color|config|dbpath|help|hookdir|gpgdir|logfile|root|version) ]]
   then
     [[ $cur = -* ]] && _arch_ptr2comp ${o#* } common ||