diff --git a/lib/include/reader.h b/lib/include/reader.h
new file mode 100644
index 0000000..5fc99d8
--- /dev/null
+++ b/lib/include/reader.h
@@ -0,0 +1,9 @@
+#ifndef READER_H
+#define READER_H
+
+#include "memory_pool.h"
+
+const expression_t *
+read_expression(memory_pool_t *pool, const char *input, int len);
+
+#endif
diff --git a/lib/reader.c b/lib/reader.c
new file mode 100644
index 0000000..9a2e20d
--- /dev/null
+++ b/lib/reader.c
@@ -0,0 +1,21 @@
+#include "reader.h"
+
+#include <ctype.h>
+#include <stddef.h>
+
+const expression_t *
+read_expression(memory_pool_t *pool, const char *input, int len)
+{
+	expression_t *result = allocate_expression(pool);
+	if (NULL == result)
+		return NULL;
+	result->is_number = true;
+	result->number = 0;
+	while (isdigit(*input)) {
+		result->number *= 10;
+		result->number += *input - '0';
+		++input;
+		++len;
+	}
+	return result;
+}
diff --git a/scripts/build.sh b/scripts/build.sh
index 890313d..72b388b 100644
--- a/scripts/build.sh
+++ b/scripts/build.sh
@@ -13,6 +13,9 @@ ar -crs build/lib.a build/memory_pool.o build/reader.o
 
 # Build tests
 clang $CFLAGS -Itests -c -o build/testing.o tests/testing.c
+clang $CFLAGS -Itests -c -o build/reader_tests.o tests/reader_tests.c
+clang $CFLAGS -o build/reader_tests \
+	  build/reader_tests.o build/lib.a build/testing.o
 
 # Build application
 clang $CFLAGS -c -o build/main.o app/main.c
diff --git a/tests/reader_tests.c b/tests/reader_tests.c
new file mode 100644
index 0000000..f50c091
--- /dev/null
+++ b/tests/reader_tests.c
@@ -0,0 +1,30 @@
+#include "reader.h"
+#include "testing.h"
+
+static memory_pool_t pool;
+
+static void input_1234_is_read_as_number_with_expected_value(void)
+{
+	init_memory_pool(&pool);
+	const expression_t *result = read_expression(&pool, "1234", 4);
+	ASSERT_NOT_NULL(result);
+	ASSERT_TRUE(result->is_number);
+	ASSERT_EQUAL(1234, result->number);
+}
+
+static void input_4321_is_read_as_number_with_expected_value(void)
+{
+	init_memory_pool(&pool);
+	const expression_t *result = read_expression(&pool, "4321", 4);
+	ASSERT_NOT_NULL(result);
+	ASSERT_TRUE(result->is_number);
+	ASSERT_EQUAL(4321, result->number);
+}
+
+int main(void)
+{
+	TESTING_BEGIN();
+	RUN_TEST(input_1234_is_read_as_number_with_expected_value);
+	RUN_TEST(input_4321_is_read_as_number_with_expected_value);
+	TESTING_END();
+}