Allow digits in variable names

Author: vanifatovvlad

README.md

@@ -63,7 +63,9 @@ Create your own `ExpressionParser` implementation to support other data types
 
 #### Variables
 
-Creation of custom variables is supported. Variable name may contains only english letters and an underscore.
+Creation of custom variables is supported.
+Variable name must starts with english letter or underscore.
+The rest of the variable name may also contains digits.
 
 #### Functions
 

Runtime/ExpressionParser.cs

@@ -62,6 +62,11 @@ select apply(f, l, r)
 
         private Parser<ExprBuilder> CreateParser()
         {
+            var letterOrUnderscore = Char(c => char.IsLetter(c) || c == '_', 
+                "letter or underscore");
+            var letterOrDigitOrUnderscore = Char(c => char.IsLetterOrDigit(c) || c == '_', 
+                "letter or digit or underscore");
+            
             var constant = (
                 from number in DecimalInvariant
                 select MakeConstant(number, Parse)
@@ -84,8 +89,9 @@ select MakeConstant(number, Parse)
 
             var variable =
             (
-                from name in Letter.Or(Char('_')).AtLeastOnce().Text()
-                select MakeVariable(name)
+                from nameHead in letterOrUnderscore.Once().Text()
+                from nameTail in letterOrDigitOrUnderscore.Many().Text()
+                select MakeVariable(nameHead + nameTail)
             ).Named("variable");
 
             Parser<ExprBuilder> expression = null;

Tests/ParserTests.cs

@@ -136,11 +136,22 @@ public void ReadmeSample()
         }
 
         [Test]
-        public void Parse_Variable_Invalid()
+        public void Parse_Variable_Names()
         {
             var context = new ExpresionContext<float>();
-            context.RegisterVariable("a", () => 1);
-            Assert.AreEqual(1, Execute("a", context));
+            context.RegisterVariable("o", () => 1);
+            context.RegisterVariable("one", () => 1);
+            context.RegisterVariable("one123", () => 1);
+            context.RegisterVariable("_one123", () => 1);
+            context.RegisterVariable("one_123", () => 1);
+            context.RegisterVariable("one123_", () => 1);
+            Assert.AreEqual(1, Execute("o", context));
+            Assert.AreEqual(1, Execute("one", context));
+            Assert.AreEqual(1, Execute("one123", context));
+            Assert.AreEqual(1, Execute("_one123", context));
+            Assert.AreEqual(1, Execute("one_123", context));
+            Assert.AreEqual(1, Execute("one123_", context));
+            Assert.Throws<ParseException>(() => Compile("123one", context));
             Assert.Throws<VariableNotDefinedException>(() => Compile("b", context));
         }