Regular Expressions Cookbook

4.8. Limit Input to Alphanumeric Characters

Problem

Your application requires that users limit their responses to one or more alphanumeric characters from the English alphabet.

Solution

With regular expressions at your disposal, the solution is dead simple. A character class can set up the allowed range of characters. With an added quantifier that repeats the character class one or more times, and anchors that bind the match to the start and end of the string, you’re good to go.

Regular expression

^[A-Z0-9]+$

Regex options: Case insensitive

Regex flavors: .NET, Java, JavaScript, PCRE, Perl, Python, Ruby

Ruby

if subject =~ /^[A-Z0-9]+$/i
    puts "Subject is alphanumeric"
else
    puts "Subject is not alphanumeric"
end

Other programming languages

See Recipes 3.4 and 3.5 for help implementing this regular expression with other programming languages.

Discussion

Let’s look at the four pieces of this regular expression one at a time:

^         # Assert position at the beginning of the string.
[A-Z0-9]  # Match a character from "A" to "Z" or from "0" to "9"...
  +       #   between one and unlimited times.
$         # Assert position at the end of the string.

Regex options: Case insensitive, free-spacing

Regex flavors: .NET, Java, PCRE, Perl, Python, Ruby

The ‹^› and ‹$› assertions at the beginning and end of the regular expression ensure that the entire input string is tested. Without them, the regex could match any part of a longer string, letting invalid characters through. The plus quantifier ‹+› repeats the preceding element one or ...