isValidLanguageTag

A simplified function that checks if a string follows the basic BCP-47 language tag pattern.

Signature

function isValidLanguageTag(tag: string): boolean;

Description

isValidLanguageTag performs a basic validation of BCP-47 language tags using a regular expression. This function is simpler and faster than the full validateLanguageTag function, but provides less thorough validation.

Note: This function uses a simplified validation approach and does not check against the registry. For complete validation, use validateLanguageTag or isValid instead.

Parameters

Parameter	Type	Required	Description
tag	string	Yes	The language tag to validate

Returns

• true if the tag matches the basic BCP-47 language tag pattern
• false if the tag does not match the pattern

Examples

import { isValidLanguageTag } from "@aleksejdix/ally-bcp47";

// Basic valid tags
isValidLanguageTag("en"); // true
isValidLanguageTag("en-US"); // true
isValidLanguageTag("zh-Hans-CN"); // true

// Invalid tags
isValidLanguageTag(""); // false - empty string
isValidLanguageTag("en-"); // false - ends with hyphen
isValidLanguageTag("123"); // false - must start with a letter

Validation Pattern

This function uses the following regular expression pattern for validation:

/^[a-z]{2,3}(-[A-Z][a-z]{3})?(-[A-Z]{2}|-[0-9]{3})?(-[A-Z0-9]{5,8})?(-[a-z0-9]{1,8})*$/

This pattern matches:

• A language code of 2-3 lowercase letters
• Optionally followed by a script code (capitalized first letter followed by 3 lowercase letters)
• Optionally followed by a region code (2 uppercase letters or 3 digits)
• Optionally followed by a variant subtag (5-8 alphanumeric characters)
• Optionally followed by any number of extension subtags (1-8 alphanumeric characters)

Implementation

function isValidLanguageTag(tag: string): boolean {
  const bcp47Regex =
    /^[a-z]{2,3}(-[A-Z][a-z]{3})?(-[A-Z]{2}|-[0-9]{3})?(-[A-Z0-9]{5,8})?(-[a-z0-9]{1,8})*$/;
  return bcp47Regex.test(tag);
}

Usage Notes

When to Use

• For simple, fast validations where precision isn't critical
• For quick filtering of obviously invalid values
• When you don't need registry validation

When to Use Other Functions Instead

• For complete validation: use validateLanguageTag or isValid
• For parsing the tag components: use parseTag
• For BCP-47 syntax validation only: use isWellFormed

Related Functions

• validateLanguageTag - For comprehensive validation
• isValid - For complete validation with a boolean result
• isWellFormed - For syntax-only validation
• parseTag - For parsing a tag into its components