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Regular Expressions Cookbook by Steven Levithan, Jan Goyvaerts

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4.14. Validate ZIP Codes

Problem

You need to validate a ZIP code (U.S. postal code), allowing both the five-digit and nine-digit (ZIP + 4) formats. The regex should match 12345 and 12345-6789, but not 1234, 123456, 123456789, or 1234-56789.

Solution

Regular expression

^[0-9]{5}(?:-[0-9]{4})?$
Regex options: None
Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

VB.NET

If Regex.IsMatch(subjectString, "^[0-9]{5}(?:-[0-9]{4})?$") Then
    Console.WriteLine("Valid ZIP code")
Else
    Console.WriteLine("Invalid ZIP code")
End If

Other programming languages

See Recipe 3.5 for help with implementing this regular expression with other programming languages.

Discussion

A breakdown of the ZIP code regular expression follows:

^           # Assert position at the beginning of the string.
[0-9]{5}    # Match a digit, exactly five times.
(?:         # Group but don't capture...
  -         #   Match a literal "-".
  [0-9]{4}  #   Match a digit, exactly four times.
)           # End the noncapturing group.
  ?         #   Repeat the preceding group between zero and one time.
$           # Assert position at the end of the string.
Regex options: Free-spacing
Regex flavors: .NET, Java, PCRE, Perl, Python, Ruby

This regex is pretty straightforward, so there isn’t much to add. A simple change that allows you to find ZIP codes within a longer input string is to replace the ^ and $ anchors with word boundaries, so you end up with \b[0-9]{5}(?:-[0-9]{4})?\b.

See Also

Recipes 4.15, 4.16, and 4.17

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