Email Validation Using Regular Expressions

by William Malone

Finding patterns in strings can be a common occurance in interface design. Ask someone to enter their email or physical address and who knows what you will get. Validating this data can be a time consuming and tedious task. There are only so many indexOf methods and if else statements one can handle. An alternative is using regular expressions.

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What are Regular Expressions?

Regular expressions describe a pattern of characters in a string. Shortened to regExp or regEx this pattern can be used (among other things) to match content of strings.

For example if I wanted to check if someone knew my name or nickname I could use the regular expression \will\ to match the characters in a string "william" and "willie". However that also includes nicknames I don't go by such as "willow" or "willis". We can expand our expression to match a little better by adding an "i" to the expression. But that will only handle "willow" not "willis". So we adjust the expression using a metacharacter similar to a conditional to: \willi(e|am)\.

As another example let's validate a phone number. Let's restrict our requirements it a local 7 digit US number to keep it simple. We define acceptable permutations as:

  • 123-3456
  • 123.3456
  • 1233456

We use the expression with the "^" metacharacter to specify the beginning of the string and nothing before. We follow that with "/d" which will require only numerals (0-9). Add the "{3}" requires 3 characters. We are currently at: \^\d{3}\.

We accept any character next so we add a dot, ".", which allows for any character and "{0,1}" which means we can have have zero to one character, but no more. Next we add \d{4} to match those last four digits. To make sure there is nothing afterward we use the metacharacter "$". Finally we have the expression: \^\d{3}.{0,1}\d{4}$\.

A Practical Example

Let's say we have a website where users are asked to enter a valid email address to continue to the next page. We are going to keep the requirements very simple for this example, such as:

  1. Must contain the @ symbol
  2. Must contain a character before the @ symbol
  3. Must contain a dot: .
  4. Must contain two characters between the @ symbol and dot
  5. Must contain two characters after the dot

Let's try to validate with JavaScript using a few standard methods:

function validEmail(str) {

    // Handle Requirement 1
    var atLoc = str.indexOf("@");
    if(atLoc == -1){
        return false;
    }
    // Handle Requirement 2
    if(atLoc < 1){
        return false;
    }
    // Handle Requirement 3
    var dotLoc = str.indexOf(".");
    if(dotLoc == -1){
        return false;
    }
    // Handle Requirement 4
    if(dotLoc - atLoc < 2){
        return false;
    }
    // Handle Requirement 5
    if(str.length - atLoc < 2){
        return false;
    }
    return true;
}

Now I'll try to make this more efficient:

function validEmail(str) {

    // Handle Requirement 1 and 2
    var atLoc = str.indexOf("@");
    if(atLoc < 1){
        return false;
    }
    // Handle Requirement 3
    var dotLoc = str.indexOf(".");
    if(dotLoc == -1){
        return false;
    }
    // Handle Requirement 4 and 5
    if(dotLoc - atLoc + str.length - atLoc < 4){
        return false;
    }
    return true;
}

validity icon

Now we try using a regular expression:

function validEmail(str) {

    // Handle All Requirements 1-5
    var regEx = /^.{1,}@.{2,}\..{2,}/;
    return regEx.match(str);
}

validity icon

Less lines of code? Yep. Less readibility? Yep.

References

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