# Practical Implementation Examples

### Example 1: User Form Validation

A script accepting user-submitted data with multiple constraints:

```
// Stack: <email> <username> <age>

// Check structure: exactly 3 inputs
OP_DEPTH OP_3 OP_EQUAL OP_VERIFY

// Validate age (1 byte, value 0-127)
OP_SIZE OP_1 OP_EQUAL OP_VERIFY

// Validate username (5-20 bytes)
OP_SWAP
OP_DUP OP_SIZE OP_5 OP_GREATERTHANOREQUAL OP_VERIFY
OP_SIZE OP_20 OP_LESSTHANOREQUAL OP_VERIFY

// Validate email (must contain @ symbol, max 50 bytes)
OP_SWAP
OP_SIZE OP_50 OP_LESSTHANOREQUAL OP_VERIFY
// Additional email format validation would follow
```

**Real-world application:** Registration systems where users submit credentials on-chain.

&#x20;

### Example 2: Payment Amount Tiers

A script that applies different validation rules based on payment amount size:

```
// Stack: <signature> <amount>

OP_SIZE OP_4 OP_LESSTHAN OP_IF
  // Small amount (< 4 bytes): relaxed validation
  <pubkey1> OP_CHECKSIG
OP_ELSE
  // Large amount (>= 4 bytes): strict validation
  OP_SIZE OP_8 OP_LESSTHANOREQUAL OP_VERIFY
  OP_DUP OP_<max_value> OP_LESSTHANOREQUAL OP_VERIFY
  <pubkey1> <pubkey2> OP_2 OP_CHECKMULTISIG
OP_ENDIF
```

**Real-world application:** Wallets implementing progressive security (simple auth for small amounts, multisig for large).

&#x20;

### Example 3: Protocol Version Detection

A script supporting multiple protocol versions based on input structure:

```
OP_DEPTH  
OP_DUP OP_2 OP_EQUAL OP_IF 
  // Version 1: 2 inputs (basic) 
  OP_DROP 
  <version1_validation> 
OP_ELSE 
  OP_DUP OP_4 OP_EQUAL OP_IF 
    // Version 2: 4 inputs (extended) 
    OP_DROP 
    <version2_validation> 
  OP_ELSE 
    // Invalid version 
    OP_RETURN 
  OP_ENDIF 
OP_ENDIF 
```

**Real-world application:** Token protocols evolving over time while maintaining backward compatibility.


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