MDL-70796 count_words: match the count from LibraOffice & MS Word

lib/moodlelib.php

@@ -8401,14 +8401,14 @@ function count_words($string) {
     $string = strip_tags($string);
     // Decode HTML entities.
     $string = html_entity_decode($string);
-    // Replace underscores (which are classed as word characters) with spaces.
-    $string = preg_replace('/_/u', ' ', $string);
-    // Remove any characters that shouldn't be treated as word boundaries.
-    $string = preg_replace('/[\'"’-]/u', '', $string);
-    // Remove dots and commas from within numbers only.
-    $string = preg_replace('/([0-9])[.,]([0-9])/u', '$1$2', $string);
 
-    return count(preg_split('/\w\b/u', $string)) - 1;
+    // Now, the word count is the number of blocks of characters separated
+    // by any sort of space. That seems to be the definition used by all other systems.
+    // To be precise about what is considered to separate words:
+    // * Anything that Unicode considers a 'Separator'
+    // * Anything that Unicode considers a 'Control character'
+    // * An em- or en- dash.
+    return count(preg_split('~[\p{Z}\p{Cc}—–]+~u', $string, -1, PREG_SPLIT_NO_EMPTY));
 }
 
 /**