Regular Expressions

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A regular expression ("regex") is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. "find and replace"-like operations. Regular expression processors are found in several search engines, search and replace dialogs of several word processors and text editors, and in the command lines of text processing utilities, such as sed and AWK. Many programming languages provide regular expression capabilities, some built-in, for example Perl, JavaScript, Ruby, AWK, and Tcl, and others via a standard library, for example .NET languages, Java, Python and C++ (since C++11). Most other languages offer regular expressions via a library.

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Validation of Email Addresses with Regular Expressions
Explain concepts important to validation of email addresses with regular expressions. Applies to most languages/tools that uses regular expressions.

1. Consider email address RFCs

2. Look at HTML5 form input element (with type=email) regex pattern

3. Try out the given pattern on some test data

4. Explain why we shouldn't be trying to match top level domains

5. Remind about server side validation

6. Remind about an expected level of knowledge

7. Mention EE Regular Expressions Topic Area

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LVL 110

Expert Comment

by:Ray Paseur
In addition to regular expressions, PHP has some built-in filters that can help.
http://php.net/manual/en/function.filter-var.php
http://php.net/manual/en/filter.filters.validate.php

Here is how I validate an email address.
<?php // demo/email_validation.php
/**
 * How to use a utility function to test for a valid email address
 * 'Valid' does not mean email will be received - parked domains are valid, but inactive
 *
 * http://php.net/manual/en/intro.filter.php
 * http://php.net/manual/en/function.checkdnsrr.php
 * https://tools.ietf.org/html/rfc5321
 *
 * See also:
 * https://www.experts-exchange.com/articles/3939/Registration-and-Email-Confirmation-in-PHP.html
 */
error_reporting(E_ALL);


function check_valid_email($email, $bogus=[], $route=TRUE)
{
    $valid = TRUE;

    // IF THE EMAIL STRING IS IMPROPERLY FORMED
    if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE) $valid = FALSE;

    // IF THE DOMAIN IS IN OUR BLOCKED LIST
    foreach ($bogus as $badguy) {
        if (stripos($email, $badguy)) $valid = FALSE;
    }

    // IF THE DOMAIN HAS NO MAIL-EXCHANGE RECORD
    if ($route) {
        $domain = explode('@', $email);
        if ( !checkdnsrr($domain[1]) ) $valid = FALSE;
    }

    return $valid;
}


// INJECTED DEPENDENCY - LIST OF BAD DOMAINS
$bogus = [ '@unknown.com', '@example.com', '@gooseball.org' ];


// DEMONSTRATE THE FUNCTION IN ACTION
$e = !empty($_GET['e']) ? $_GET['e'] : NULL;
if ($e)
{
    if (check_valid_email($e, $bogus))
    {
        echo "<br/>VALID: $e" . PHP_EOL;
    }
    else
    {
        echo "<br/>BOGUS: $e" . PHP_EOL;
    }
}


// END OF PROCESSING - CREATE THE FORM USING HEREDOC NOTATION
$form = <<<ENDFORM
<form>
TEST A STRING FOR A VALID EMAIL ADDRESS:
<input name="e" value="$e" />
<input type="submit" />
</form>
ENDFORM;

echo $form;

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Matching Tagged Data with PHP Regular Expressions
Learn how to match and substitute tagged data using PHP regular expressions. Demonstrated on Windows 7, but also applies to other operating systems. Demonstrated technique applies to PHP (all versions) and Firefox, but very similar techniques will work in other browsers.

1. Important note about parsing tagged data

2. Show how to find an element on an HTML page using Firefox developer tools

3. Show how to capture the tagged data using the PHP function preg_match

4. Show how to separate the tags from the enclosed data

5. Show how to use the PHP preg_replace function to replace a value with a new value

1

Regular Expressions

6K

Solutions

7

Articles & Videos

5K

Contributors

A regular expression ("regex") is a sequence of characters that define a search pattern, mainly for use in pattern matching with strings, or string matching, i.e. "find and replace"-like operations. Regular expression processors are found in several search engines, search and replace dialogs of several word processors and text editors, and in the command lines of text processing utilities, such as sed and AWK. Many programming languages provide regular expression capabilities, some built-in, for example Perl, JavaScript, Ruby, AWK, and Tcl, and others via a standard library, for example .NET languages, Java, Python and C++ (since C++11). Most other languages offer regular expressions via a library.