diff --git a/src/lexical_analysis/mod.rs b/src/lexical_analysis/mod.rs
index c2b223ac..a644f742 100644
--- a/src/lexical_analysis/mod.rs
+++ b/src/lexical_analysis/mod.rs
@@ -187,9 +187,20 @@ fn parse_identifier(parsing: &str) -> Option<Token> {
 }
 
 fn is_valid_identifier(verifying: &str) -> bool {
-    verifying.chars().all(is_valid_identifier_character)
+    let mut characters = verifying.chars();
+
+    let starts_valid = match characters.next() {
+        Some(character) => is_valid_identifier_start_character(character),
+        None => false,
+    };
+
+    starts_valid && characters.all(is_valid_identifier_character)
 }
 
-fn is_valid_identifier_character(character: char) -> bool {
+fn is_valid_identifier_start_character(character: char) -> bool {
     character.is_alphabetic() || character == '_'
 }
+
+fn is_valid_identifier_character(character: char) -> bool {
+    is_valid_identifier_start_character(character) || character.is_ascii_digit()
+}
diff --git a/src/tests/identifier_expression/mod.rs b/src/tests/identifier_expression/mod.rs
index bd3e6c37..7b34807d 100644
--- a/src/tests/identifier_expression/mod.rs
+++ b/src/tests/identifier_expression/mod.rs
@@ -42,6 +42,10 @@ use super::*;
     case(
         "let is_file = FALSE; let is_directory = !is_file;\nis_directory",
         "identifier_expression_case_15"
+    ),
+    case(
+        "let x1 = 5;\nlet arg2 = 10;\nx1 + arg2",
+        "identifier_expression_case_16"
     )
 )]
 fn identifier_expression_cases(code: &str, snapshot_name: &str) {}
diff --git a/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_environment.snap b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_environment.snap
new file mode 100644
index 00000000..91bf3d88
--- /dev/null
+++ b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_environment.snap
@@ -0,0 +1,17 @@
+---
+source: src/tests/identifier_expression/mod.rs
+expression: evaluator
+---
+Evaluator {
+    environment: Environment {
+        variables: {
+            "arg2": Integer {
+                value: 10,
+            },
+            "x1": Integer {
+                value: 5,
+            },
+        },
+        sub_environment: None,
+    },
+}
diff --git a/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_evaluation.snap b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_evaluation.snap
new file mode 100644
index 00000000..8b824dbc
--- /dev/null
+++ b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_evaluation.snap
@@ -0,0 +1,7 @@
+---
+source: src/tests/identifier_expression/mod.rs
+expression: evaluation
+---
+Integer {
+    value: 15,
+}
diff --git a/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_lexical_analysis.snap b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_lexical_analysis.snap
new file mode 100644
index 00000000..b9abd24d
--- /dev/null
+++ b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_lexical_analysis.snap
@@ -0,0 +1,31 @@
+---
+source: src/tests/identifier_expression/mod.rs
+expression: tokens
+---
+[
+    Let,
+    Identifier {
+        literal: "x1",
+    },
+    Assign,
+    Integer {
+        literal: 5,
+    },
+    SemiColon,
+    Let,
+    Identifier {
+        literal: "arg2",
+    },
+    Assign,
+    Integer {
+        literal: 10,
+    },
+    SemiColon,
+    Identifier {
+        literal: "x1",
+    },
+    Plus,
+    Identifier {
+        literal: "arg2",
+    },
+]
diff --git a/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_syntax_analysis.snap b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_syntax_analysis.snap
new file mode 100644
index 00000000..df6b14cd
--- /dev/null
+++ b/src/tests/identifier_expression/snapshots/monkey_interpreter__tests__identifier_expression__test_identifier_expression_case_16_syntax_analysis.snap
@@ -0,0 +1,33 @@
+---
+source: src/tests/identifier_expression/mod.rs
+expression: abstract_syntax_tree
+---
+[
+    Statement {
+        statement: Let {
+            identifier: "x1",
+            expression: Integer {
+                literal: 5,
+            },
+        },
+    },
+    Statement {
+        statement: Let {
+            identifier: "arg2",
+            expression: Integer {
+                literal: 10,
+            },
+        },
+    },
+    Expression {
+        expression: Infix {
+            left_hand: Identifier {
+                identifier: "x1",
+            },
+            operator: Plus,
+            right_hand: Identifier {
+                identifier: "arg2",
+            },
+        },
+    },
+]