diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index d19b5cd7492..b9368f7492f 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -132,6 +132,14 @@ Also, please bear the following coding guidelines in mind:
   expansions will be unexpectedly performed, which becomes a vulnerability.  In
   the latter case, checks by shellcheck and shfmt will not be performed inside
   `'...'`.  Also, `_comp_compgen_split` is `IFS`-safe.
+  
+  Avoid using `_comp_compgen -- -G "pattern"` to generate completions.  The
+  result is not filtered by the current word `cur` due to the Bash design of
+  `compgen`.  Also, this cannot be used to generate filenames with a specified
+  extension because the `-G` specification only generates the matching
+  filepaths in the current directory.  It does not look into subdirectories
+  even when `$cur` implies completion in a subdirectory.  One can instead use
+  `_comp_compgen -- -f -X '!pattern'`.
 
 - When completing available options, offer only the most descriptive
   ones as completion results if there are multiple options that do the
diff --git a/completions/feh b/completions/feh
index d9550519070..57a23c4bf83 100644
--- a/completions/feh
+++ b/completions/feh
@@ -29,13 +29,13 @@ _comp_cmd_feh()
             fi
             local font_path
             # font_path="$(imlib2-config --prefix 2>/dev/null)/share/imlib2/data/fonts"
-            # _comp_compgen -C "$font_path" -- -f -X "!*.@([tT][tT][fF])" -S /
+            # _comp_compgen -C "$font_path" -- -f -X "!*.[tT][tT][fF]" -S /
             for ((i = ${#words[@]} - 2; i > 0; i--)); do
                 if [[ ${words[i]} == -@(C|-fontpath) ]]; then
                     font_path="${words[i + 1]}"
                     if [[ -d $font_path ]]; then
                         _comp_compgen -aC "$font_path" -- \
-                            -f -X "!*.@([tT][tT][fF])" -S /
+                            -f -X "!*.[tT][tT][fF]" -S /
                     fi
                 fi
             done
diff --git a/completions/mdtool b/completions/mdtool
index cd179e5f436..22a409e5985 100644
--- a/completions/mdtool
+++ b/completions/mdtool
@@ -21,10 +21,10 @@ _comp_cmd_mdtool()
                 #if [[ "$prev" == *: ]]; then
                 #   case $prev in
                 #       @(--p:|--project:))
-                #           _comp_compgen -- -f -G "*.mdp"
+                #           _comp_compgen -- -f -X '!*.mdp'
                 #           ;;
                 #       @(--f:|--buildfile:))
-                #           _comp_compgen -- -f -G "*.mdp" -G "*.mds"
+                #           _comp_compgen -- -f -X '!*.md[ps]'
                 #           ;;
                 #   esac
                 #fi
@@ -32,7 +32,7 @@ _comp_cmd_mdtool()
                 ;;
             "generate-makefiles")
                 compopt -o filenames
-                _comp_compgen -- -o filenames -G"*.mds"
+                _comp_compgen -- -o filenames -f -X '!*.mds'
                 if [[ $prev == *mds ]]; then
                     _comp_compgen -- -W '--simple-makefiles --s --d:'
                 fi
diff --git a/completions/rcs b/completions/rcs
index aab018e2473..bbf9c6b763d 100644
--- a/completions/rcs
+++ b/completions/rcs
@@ -5,7 +5,7 @@ _comp_cmd_rcs()
     local cur prev words cword comp_args
     _comp_initialize -- "$@" || return
 
-    local file dir i
+    local file dir
 
     file=${cur##*/}
     dir=${cur%/*}
@@ -21,8 +21,11 @@ _comp_cmd_rcs()
         COMPREPLY[i]=$dir$file
     done
 
-    _comp_compgen -aR -- -G "$dir/$file*,v"
+    local files
+    _comp_expand_glob files '"$dir/$file"*,v' &&
+        _comp_compgen -aR -- -W '"${files[@]}"'
 
+    local i
     for i in ${!COMPREPLY[*]}; do
         COMPREPLY[i]=${COMPREPLY[i]%,v}
     done
diff --git a/completions/upgradepkg b/completions/upgradepkg
index f359e9add19..0012f6282c3 100644
--- a/completions/upgradepkg
+++ b/completions/upgradepkg
@@ -15,7 +15,7 @@ _comp_cmd_upgradepkg()
         cur="${cur#*%}"
         local nofiles=""
         compopt -o filenames
-        _comp_compgen -- -P "$prev%" -f -X "!*.@(t[bgxl]z)" || nofiles=set
+        _comp_compgen -- -P "$prev%" -f -X "!*.t[bgxl]z" || nofiles=set
         _comp_compgen -a -- -P "$prev%" -S '/' -d
         [[ $nofiles ]] && compopt -o nospace
         return
diff --git a/completions/valgrind b/completions/valgrind
index 7043aa32fdb..12782922572 100644
--- a/completions/valgrind
+++ b/completions/valgrind
@@ -7,7 +7,7 @@ _comp_cmd_valgrind()
 
     local i
     for ((i = 1; i <= cword; i++)); do
-        if [[ ${words[i]} != @([-=])* ]]; then
+        if [[ ${words[i]} != [-=]* ]]; then
             _comp_command_offset $i
             return
         fi
diff --git a/test/runLint b/test/runLint
index 550995de147..d011736b18f 100755
--- a/test/runLint
+++ b/test/runLint
@@ -52,6 +52,9 @@ gitgrep '(?<!command)'"$cmdstart"'(grep|ls|sed|cd)(\s|$)' \
 gitgrep '(?<!command)'"$cmdstart"'awk(\s|$)' \
     'invoke awk through "_comp_awk"'
 
+gitgrep '@\([^()|$]+\)' \
+    '@(...) may not be needed when ... does not contain |.'
+
 #------------------------------------------------------------------------------
 # Bash pitfalls/styles/compatibilities (which are not detected by shellcheck)