diff --git a/src/Layout.cpp b/src/Layout.cpp
index 7f4925a8c3..b2d4d2e273 100644
--- a/src/Layout.cpp
+++ b/src/Layout.cpp
@@ -23,8 +23,9 @@
 #include "support/lassert.h"
 #include "support/lstrings.h"
 #include "support/Messages.h"
-
 #include "support/regex.h"
+#include "support/textutils.h"
+
 
 using namespace std;
 using namespace lyx::support;
@@ -997,7 +998,7 @@ string Layout::defaultCSSClass() const
 	docstring::const_iterator en = name().end();
 	for (; it != en; ++it) {
 		char_type const c = *it;
-		if (c >= 0x80 || !isalnum(c)) {
+		if (!isAlphaASCII(c)) {
 			if (d.empty())
 				// make sure we don't start with an underscore,
 				// as that sometimes causes problems.
diff --git a/src/insets/InsetRef.cpp b/src/insets/InsetRef.cpp
index aa0bcb573e..8324104eed 100644
--- a/src/insets/InsetRef.cpp
+++ b/src/insets/InsetRef.cpp
@@ -28,6 +28,7 @@
 #include "support/docstream.h"
 #include "support/gettext.h"
 #include "support/lstrings.h"
+#include "support/textutils.h"
 
 using namespace lyx::support;
 using namespace std;
@@ -114,8 +115,8 @@ docstring InsetRef::getFormattedCmd(docstring const & ref,
 	// make sure the prefix is legal for a latex command
 	int const len = prefix.size();
 	for (int i = 0; i < len; i++) {
-		char_type c = prefix[i];
-		if (c >= 0x80 || !isalpha(prefix[i])) {
+		char_type const c = prefix[i];
+		if (!isAlphaASCII(c)) {
 			LYXERR0("Prefix `" << prefix << "' is invalid for LaTeX.");
 			// restore the label
 			label = ref;
diff --git a/src/output_xhtml.cpp b/src/output_xhtml.cpp
index 40759cea40..d8299ce5c4 100644
--- a/src/output_xhtml.cpp
+++ b/src/output_xhtml.cpp
@@ -32,6 +32,7 @@
 #include "support/debug.h"
 #include "support/lassert.h"
 #include "support/lstrings.h"
+#include "support/textutils.h"
 
 #include <vector>
 
@@ -130,8 +131,7 @@ docstring cleanAttr(docstring const & str)
 	docstring::const_iterator en = str.end();
 	for (; it != en; ++it) {
 		char_type const c = *it;
-		bool const is_alnum = c < 0x80 && isalnum(c);
-		newname += is_alnum ? c : char_type('_');
+		newname += isAlnumASCII(c) ? c : char_type('_');
 	}
 	return newname;	
 }
diff --git a/src/support/lstrings.cpp b/src/support/lstrings.cpp
index daa862747c..74950b397f 100644
--- a/src/support/lstrings.cpp
+++ b/src/support/lstrings.cpp
@@ -161,6 +161,14 @@ bool isDigitASCII(char_type c)
 	return '0' <= c && c <= '9';
 }
 
+
+bool isAlnumASCII(char_type c)
+{
+	return ('0' <= c && c <= '9')
+		|| ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z');
+}
+
+
 namespace support {
 
 int compare_no_case(docstring const & s, docstring const & s2)
diff --git a/src/support/textutils.h b/src/support/textutils.h
index e1a3063d82..99594f32c3 100644
--- a/src/support/textutils.h
+++ b/src/support/textutils.h
@@ -47,6 +47,9 @@ bool isDigit(char_type c);
 /// return whether \p c is a digit in the ASCII range
 bool isDigitASCII(char_type c);
 
+/// return whether \p c is alpha or a digit in the ASCII range
+bool isAlnumASCII(char_type c);
+
 } // namespace lyx
 
 #endif // TEXTUTILS_H