When working with phone numbers in software applications, it’s crucial to identify and filter out invalid numbers to ensure reliable communication and data integrity. Invalid numbers may cause failed deliveries, errors in processing, or poor user experience. In code, identifying invalid phone numbers involves a combination of format validation, length checks, country-specific rules, and sometimes external API verification. Here’s how developers commonly detect invalid phone numbers programmatically.
1. Basic Format Validation Using Regular Expressions
The simplest method to check phone numbers is to use regular expressions (regex). Regex allows you to verify if the input matches a general phone number pattern, such as:
Contains only digits, spaces, parentheses, dashes, or plus signs
Starts with a plus sign followed by country code (optional)
Has the correct number of digits
Example regex for international phone numbers might recent mobile phone number data look like this (simplified):
regex
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^\+?[0-9\s\-()]{7,15}$
This checks if the phone number:
Starts with an optional “+”
Contains 7 to 15 characters that are digits, spaces, hyphens, or parentheses
While regex is good for initial format screening, it can’t verify if the number is valid or assigned.
2. Length and Numeric Checks
Phone numbers vary in length by country but usually fall within known minimum and maximum lengths. Code can check:
If the number has too few or too many digits after stripping non-numeric characters
If the number contains only numeric characters after cleanup
For example, a U.S. phone number typically has 10 digits (excluding country code), so a length check ensures it’s not shorter or longer than expected.
3. Country Code and Number Plan Validation
Phone numbers include a country code prefix (e.g., +1 for the U.S., +91 for India). Code can:
Extract the country code from the number
Verify if the country code is valid using a predefined list of country codes
Check if the subscriber number fits the expected pattern for that country’s numbering plan
This requires either hardcoded rules per country or using specialized libraries.
4. Using Libraries Like Google’s libphonenumber
The most reliable method is to use established phone number validation libraries, such as Google’s libphonenumber, which is available in many programming languages (Java, JavaScript, Python, etc.). It offers:
Parsing phone numbers according to international standards
Validating if the number is possible (correct length, structure)
Validating if the number is valid (assigned by telecom authorities)
Formatting numbers in standardized ways (E.164)
Detecting number types (mobile, landline, toll-free)
Example in Python:
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from phonenumbers import parse, is_valid_number
number = "+14155552671"
parsed_number = parse(number, None)
if is_valid_number(parsed_number):
print("Valid number")
else:
print("Invalid number")
This method handles edge cases and country-specific rules much better than regex.
5. API-Based Validation
For real-time validation or more accurate checks, apps use phone validation APIs (e.g., Twilio Lookup, NumVerify). These APIs:
Confirm if the number is currently assigned and active
Identify carrier and line type
Detect disconnected or spoofed numbers
APIs require network calls but provide the highest confidence.
6. Summary of Common Checks
To identify invalid phone numbers in code:
Strip all non-numeric characters
Check length against expected ranges
Validate country code presence and correctness
Use libraries (libphonenumber) for detailed validation
Optionally, call validation APIs for real-time checks
Conclusion
Identifying invalid phone numbers programmatically combines basic format and length checks with more sophisticated parsing and validation tools like libphonenumber and external APIs. Implementing these methods helps maintain data quality, reduce errors, and improve communication reliability in applications.