diff --git a/README.md b/README.md
index 8db7611..749ad37 100644
--- a/README.md
+++ b/README.md
@@ -255,11 +255,10 @@ TinyExpr parses the following grammar:
 
 In addition, whitespace between tokens is ignored.
 
-Valid variable names consist of a lower case letter followed by any combination
-of: lower case letters *a* through *z*, the digits *0* through *9*, and
-underscore. Constants can be integers, decimal numbers, or in scientific
-notation (e.g.  *1e3* for *1000*). A leading zero is not required (e.g. *.5*
-for *0.5*)
+Valid variable names consist of a letter followed by any combination of:
+letters, the digits *0* through *9*, and underscore. Constants can be integers,
+decimal numbers, or in scientific notation (e.g.  *1e3* for *1000*). A leading
+zero is not required (e.g. *.5* for *0.5*)
 
 
 ## Functions supported
diff --git a/smoke.c b/smoke.c
index 8accfe6..86d6dff 100644
--- a/smoke.c
+++ b/smoke.c
@@ -174,8 +174,9 @@ void test_syntax() {
         {"1*2(+4", 4},
         {"1*2(1+4", 4},
         {"a+5", 1},
-        {"A+5", 1},
-        {"Aa+5", 1},
+        {"!+5", 1},
+        {"_a+5", 1},
+        {"#a+5", 1},
         {"1^^5", 3},
         {"1**5", 3},
         {"sin(cos5", 8},
diff --git a/tinyexpr.c b/tinyexpr.c
index 4c0971e..05774ff 100755
--- a/tinyexpr.c
+++ b/tinyexpr.c
@@ -243,7 +243,7 @@ void next_token(state *s) {
             s->type = TOK_NUMBER;
         } else {
             /* Look for a variable or builtin function call. */
-            if (s->next[0] >= 'a' && s->next[0] <= 'z') {
+            if (isalpha(s->next[0])) {
                 const char *start;
                 start = s->next;
                 while (isalpha(s->next[0]) || isdigit(s->next[0]) || (s->next[0] == '_')) s->next++;