diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..cb77d18
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+*.dSYM
diff --git a/.vscode/launch.json b/.vscode/launch.json
new file mode 100644
index 0000000..02fddc7
--- /dev/null
+++ b/.vscode/launch.json
@@ -0,0 +1,20 @@
+{
+    // Use IntelliSense to learn about possible attributes.
+    // Hover to view descriptions of existing attributes.
+    // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "fsm",
+            "type": "cppdbg",
+            "request": "launch",
+            "program": "${workspaceFolder}/fsm",
+            "args": [],
+            "stopAtEntry": false,
+            "cwd": "${fileDirname}",
+            "environment": [],
+            "externalConsole": false,
+            "MIMode": "lldb"
+        }
+    ]
+}
\ No newline at end of file
diff --git a/fsm b/fsm
new file mode 100755
index 0000000..70a7254
Binary files /dev/null and b/fsm differ
diff --git a/fsm.c b/fsm.c
new file mode 100644
index 0000000..2f58270
--- /dev/null
+++ b/fsm.c
@@ -0,0 +1,167 @@
+#include <ctype.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#define ARR_LEN(X) (sizeof(X) / sizeof(X[0]))
+
+typedef enum {
+  START_STATE,
+  DECIMAL_STATE,
+  NUM_STATE,
+  FRACTION_STATE,
+  EXPONENT_STATE,
+  EXP_SIGN_STATE,
+  POWER_STATE,
+  END_STATE,
+  INVALID_STATE,
+} states;
+
+states handle_start(char input);
+states handle_decimal(char input);
+states handle_num(char input);
+states handle_fraction(char input);
+states handle_exponent(char input);
+states handle_exp_sign(char input);
+states handle_power(char input);
+
+states state_machine(states current, char input);
+
+bool check_number(const char *num);
+
+const char *inputs[] = {
+    "0000000", // INVALID
+    "01", // INVALID
+    "f0", // INVALID
+    "1f", // INVALID
+    "1.f", // INVALID
+    "1.0f", // INVALID
+    "8987", // VALID
+    "9.5", // VALID
+    "    7", // VALID
+    "  7.8", // VALID
+    "732.9834e",   // INVALID
+    "732.9834e-",  // INVALID
+    "732.9834e+",  // INVALID
+    "732.9834e-g", // INVALID
+    "732.9834e-8", // VALID
+    "732.9834e+8", // VALID
+};
+
+int main(int argc, char *argv[]) {
+  for (int i = 0; i < ARR_LEN(inputs); ++i) {
+    printf("%s is %s\n", inputs[i],
+           check_number(inputs[i]) ? "VALID" : "INVALID");
+  }
+
+  return EXIT_SUCCESS;
+}
+
+states handle_start(char input) {
+  if (isspace(input)) {
+    return START_STATE;
+  } else if (input == '0') {
+    return DECIMAL_STATE;
+  } else if (isdigit(input)) {
+    return NUM_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states handle_decimal(char input) {
+  if (input == '.') {
+    return FRACTION_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states handle_num(char input) {
+  if (isdigit(input)) {
+    return NUM_STATE;
+  } else if (input == '.') {
+    return FRACTION_STATE;
+  } else if (isspace(input)) {
+    return END_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states handle_fraction(char input) {
+  if (isdigit(input)) {
+    return FRACTION_STATE;
+  } else if (isspace(input)) {
+    return END_STATE;
+  } else if (input == 'e' || input == 'E') {
+    return EXPONENT_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states handle_exponent(char input) {
+  if (isdigit(input)) {
+    return POWER_STATE;
+  } else if (input == '+' || input == '-') {
+    return EXP_SIGN_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states handle_exp_sign(char input) {
+  if (isdigit(input)) {
+    return POWER_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states handle_power(char input) {
+  if (isdigit(input)) {
+    return POWER_STATE;
+  } else if (isspace(input)) {
+    return END_STATE;
+  }
+
+  return INVALID_STATE;
+}
+
+states state_machine(states current, char input) {
+  switch (current) {
+  case START_STATE:
+    return handle_start(input);
+  case DECIMAL_STATE:
+    return handle_decimal(input);
+  case NUM_STATE:
+    return handle_num(input);
+  case FRACTION_STATE:
+    return handle_fraction(input);
+  case EXPONENT_STATE:
+    return handle_exponent(input);
+  case EXP_SIGN_STATE:
+    return handle_exp_sign(input);
+  case POWER_STATE:
+    return handle_power(input);
+  case END_STATE:
+  case INVALID_STATE:
+    return current;
+  }
+}
+
+bool check_number(const char *num) {
+  states current = START_STATE;
+
+  for (const char *c = &(num[0]); *c != '\0'; ++c) {
+    current = state_machine(current, *c);
+
+    if (current == INVALID_STATE) {
+      return false;
+    }
+  }
+
+  return current == END_STATE || current == NUM_STATE ||
+         current == FRACTION_STATE || current == POWER_STATE;
+}