Phone number libraries rely heavily on metadata databases containing up-to-date information about global numbering plans. These metadata files include:
-
Country Calling Codes: The international prefix for each country (e.g., +1 for the US/Canada, +91 for India).
-
Number Lengths: Valid lengths for national and international numbers.
-
Number Patterns: Regular expressions that match valid number formats for different regions.
-
Formatting Rules: How to display numbers based on country standards (spaces, hyphens, parentheses).
-
Carrier Information: Optional data to associate phone numbers with telecom carriers.
When you input a phone number, the library parses the string, identifies the country code (or assumes a default), and then applies the relevant rules to verify and format it correctly.
For example, libphonenumber’s parsing algorithm works like this:
-
Normalization: Remove non-digit characters except the leading “+” sign.
-
Country Identification: If the number starts with “+”, match the subsequent digits against known country codes.
-
Validation: Apply country-specific regex patterns to confirm the number’s validity.
-
Formatting: Convert the number to desired formats (E.164, international, or national).
-
Additional Metadata: Optionally determine if the number is fixed-line, mobile, or toll-free.
Handling Edge Cases and Challenges
Even with robust libraries, developers face edge cases:
-
Local Dialing vs International: Users might input local formats without country codes, causing ambiguity. Prompt users to select their country or auto-detect location if possible.
-
Number Portability: Numbers can be ported between carriers and regions, so carrier data might not always be accurate.
-
Temporary and Disposable Numbers: Some users register with temporary phone numbers to avoid verification. Libraries can’t always detect these, so consider third-party services for phone number intelligence.
-
Changing Number Plans: Countries occasionally change numbering rules. Keep your libraries and metadata updated regularly.
-
Formatting in Multilingual Interfaces: Some languages use different scripts or number representations; ensure your app handles Unicode input correctly.