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# Validation
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ULID format validation to ensure proper structure, length, and character encoding according to the ULID specification.
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## Capabilities
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### ULID Validation
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Validates whether a string is a properly formatted ULID.
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```typescript { .api }
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/**
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 * Check if a ULID is valid
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 * @param id - The string to validate
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 * @returns True if the string is a valid ULID, false otherwise
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 */
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function isValid(id: string): boolean;
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```
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**Usage Examples:**
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```typescript
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import { isValid, ulid } from "ulid";
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// Validate generated ULID
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const id = ulid();
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console.log(isValid(id)); // true
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// Validate existing ULIDs
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console.log(isValid("01HNZX8JGFACFA36RBXDHEQN6E")); // true
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console.log(isValid("01ARYZ6S41TSV4RRFFQ69G5FAV")); // true
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// Invalid cases
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console.log(isValid("")); // false - empty string
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console.log(isValid("invalid")); // false - wrong length
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console.log(isValid("01HNZX8JGFACFA36RBXDHEQN6")); // false - too short
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console.log(isValid("01HNZX8JGFACFA36RBXDHEQN6EX")); // false - too long
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console.log(isValid("01HNZX8JGFACFA36RBXDHEQN6U")); // false - invalid character 'U'
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console.log(isValid("81HNZX8JGFACFA36RBXDHEQN6E")); // false - invalid first character
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// Use in conditional logic
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function processULID(id: string) {
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  if (!isValid(id)) {
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    throw new Error(`Invalid ULID: ${id}`);
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  }
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  // Process valid ULID
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  return id.toLowerCase();
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}
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// Filter valid ULIDs from array
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const ids = ["01HNZX8JGFACFA36RBXDHEQN6E", "invalid", "01ARYZ6S41TSV4RRFFQ69G5FAV"];
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const validIds = ids.filter(isValid);
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console.log(validIds); // ["01HNZX8JGFACFA36RBXDHEQN6E", "01ARYZ6S41TSV4RRFFQ69G5FAV"]
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```
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**Validation Rules:**
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The `isValid` function checks that the input string:
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1. **Is a string type** - Non-string values return false
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2. **Has correct length** - Must be exactly 26 characters
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3. **Uses valid characters** - All characters must be from Crockford Base32 alphabet
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4. **Has valid first character** - Must be 0-7 (ensures timestamp doesn't exceed maximum)
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**Crockford Base32 Alphabet:**
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Valid characters: `0123456789ABCDEFGHJKMNPQRSTVWXYZ`
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Excluded characters: `I`, `L`, `O`, `U` (to avoid confusion with similar-looking characters)
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**Case Sensitivity:**
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The validation function is case-insensitive. Both uppercase and lowercase characters are accepted:
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```typescript
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console.log(isValid("01hnzx8jgfacfa36rbxdheqn6e")); // true - lowercase
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console.log(isValid("01HNZX8JGFACFA36RBXDHEQN6E")); // true - uppercase  
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console.log(isValid("01HnZx8JgFaCfA36RbXdHeQn6E")); // true - mixed case
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```
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**Performance Notes:**
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- The validation is performed using regular expressions for efficiency
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- Case conversion is handled internally without modifying input
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- No exceptions are thrown - invalid inputs return `false`