diff --git a/phpBB/phpbb/profilefields/type/type_string_common.php b/phpBB/phpbb/profilefields/type/type_string_common.php
index ff33a7b49c..f5e1992044 100644
--- a/phpBB/phpbb/profilefields/type/type_string_common.php
+++ b/phpBB/phpbb/profilefields/type/type_string_common.php
@@ -18,11 +18,11 @@ abstract class type_string_common extends type_base
 	protected $validation_options = array(
 		'CHARS_ANY'			=> '.*',
 		'NUMBERS_ONLY'		=> '[0-9]+',
-		'ALPHA_ONLY'		=> '[\w]+',
-		'ALPHA_UNDERSCORE'	=> '[\w_]+',
-		'ALPHA_DOTS'        => '[\w.]+',
-		'ALPHA_SPACERS'		=> '[\w\x20_+\-\[\]]+',
-		'ALPHA_PUNCTUATION' => '[a-zA-Z][\w\.,\-_]+',
+		'ALPHA_ONLY'		=> '[a-zA-Z0-9]+',
+		'ALPHA_UNDERSCORE'	=> '[\w]+',
+		'ALPHA_DOTS'        => '[a-zA-Z0-9.]+',
+		'ALPHA_SPACERS'		=> '[\w\x20+\-\[\]]+',
+		'ALPHA_PUNCTUATION' => '[a-zA-Z][\w\.,\-]+',
 		'LETTER_NUM_ONLY'			=> '[\p{Lu}\p{Ll}0-9]+',
 		'LETTER_NUM_UNDERSCORE'		=> '[\p{Lu}\p{Ll}0-9_]+',
 		'LETTER_NUM_DOTS'			=> '[\p{Lu}\p{Ll}0-9.]+',
diff --git a/tests/profilefields/type_string_test.php b/tests/profilefields/type_string_test.php
index a7be087fb5..0417afbfab 100644
--- a/tests/profilefields/type_string_test.php
+++ b/tests/profilefields/type_string_test.php
@@ -133,37 +133,49 @@ class phpbb_profilefield_type_string_test extends phpbb_test_case
 			),
 			array(
 				'ö äö äö ä',
-				array('field_validation' => '[\w]+'),
+				array('field_validation' => '[a-zA-Z0-9]+'),
 				'FIELD_INVALID_CHARS_ALPHA_ONLY-field',
 				'Required field should reject UTF-8 in alpha only field',
 			),
+			array(
+				'a_abc',
+				array('field_validation' => '[a-zA-Z0-9]+'),
+				'FIELD_INVALID_CHARS_ALPHA_ONLY-field',
+				'Required field should reject underscore in alpha only field',
+			),
 			array(
 				'Hello',
-				array('field_validation' => '[\w]+'),
+				array('field_validation' => '[a-zA-Z0-9]+'),
 				false,
 				'Required field should accept a characters only field',
 			),
 			array(
 				'Valid.Username123',
-				array('field_validation' => '[\w.]+'),
+				array('field_validation' => '[a-zA-Z0-9.]+'),
 				false,
 				'Required field should accept a alphanumeric field with dots',
 			),
 			array(
 				'Invalid.,username123',
-				array('field_validation' => '[\w.]+'),
+				array('field_validation' => '[a-zA-Z0-9.]+'),
 				'FIELD_INVALID_CHARS_ALPHA_DOTS-field',
 				'Required field should reject field with comma',
 			),
+			array(
+				'Invalid._username123',
+				array('field_validation' => '[a-zA-Z0-9.]+'),
+				'FIELD_INVALID_CHARS_ALPHA_DOTS-field',
+				'Required field should reject field with underscore',
+			),
 			array(
 				'skype.test.name,_this',
-				array('field_validation' => '[a-zA-Z][\w\.,\-_]+'),
+				array('field_validation' => '[a-zA-Z][\w\.,\-]+'),
 				false,
 				'Required field should accept alphanumeric field with punctuations',
 			),
 			array(
 				'1skype.this.should.faila',
-				array('field_validation' => '[a-zA-Z][\w\.,\-_]+'),
+				array('field_validation' => '[a-zA-Z][\w\.,\-]+'),
 				'FIELD_INVALID_CHARS_ALPHA_PUNCTUATION-field',
 				'Required field should reject field having invalid input for the given validation',
 			),