Fix: Patterns matching the empty string rejected

As observed in #86, fingers accepts a pattern accepting an empty string, such as `(token)?`, and this leads the `gawk` subprocess to loop indefinitely, allocated memory until it's eventually killed by the kernel.
This pr rejects such patterns, printing an error message.

README.md

@@ -128,7 +128,8 @@ set -g @fingers-pattern-1 'yolo'
 set -g @fingers-pattern-50 'whatever'
 ```
 
-Patterns are case insensitive, and grep's extended syntax ( ERE ) should be used.
+Patterns are case insensitive, and grep's extended syntax ( ERE ) should be used. Patterns
+matching the empty string are disallowed.
 `man grep` for more info.
 
 If the introduced regexp contains an error, an error will be shown when

scripts/config.sh

@@ -7,7 +7,6 @@ TMUX_PRINTER="$CONF_CURRENT_DIR/../vendor/tmux-printer/tmux-printer"
 
 declare -A fingers_defaults
 
-# TODO empty patterns are invalid
 function check_pattern() {
   echo "beep beep" | grep -e "$1" 2> /dev/null
 
@@ -18,6 +17,14 @@ function check_pattern() {
   fi
 }
 
+function check_matches_empty_string() {
+  if ! grep -e "$1" <<< "" >/dev/null; then
+    echo 0
+  else
+    echo 1
+  fi
+}
+
 function envify() {
   echo $1 | tr '[:lower:]' '[:upper:]' | sed "s/-/_/g"
 }
@@ -109,6 +116,14 @@ for pattern in "${PATTERNS_LIST[@]}" ; do
     PATTERNS_LIST[$i]="nope{4000}"
   fi
 
+  pattern_matches_empty=$(check_matches_empty_string "$pattern")
+
+  if [[ $pattern_matches_empty == 0 ]]; then
+    display_message "fingers-error: user defined pattern $pattern matched the empty string" 5000
+    PATTERNS_LIST[$i]="nope{4000}"
+  fi
+
+
   i=$((i + 1))
 done