/^(\w+) is a programmer. \1's specialty is programming.$/

If you try to match the preceding regex against the following sentence:

Marco is a programmer. Marco's specialty is programming.

Everything will work fine. However, if you try it against this sentence:

Marco is a programmer. John's specialty is programming.

The regex compiler will not return a match because the reference won't match.

To give you an idea of how powerful PCREs are and why it's worth trying to learn them, let me give you an alternative to the simple one-line expression using POSIX:

<?php

    $s = 'Marco is a programmer. Marco\'s specialty is programming.';

    if (ereg ('^([[:alpha:]]+) is a programmer', $s, $matches)) {
      if (ereg ('([[:alpha:]]+)\'s specialty is programming., $s, $matches2)) {
        if ($matches[1] === $matches[1]) {
          echo "MATCH\n";
        } else {
          echo "NO MATCH\n";
       } else {
          echo "NO MATCH\n";
    } else {
      echo "NO MATCH\n";
      }
?>

Now, this is a simple example, and the POSIX solution is definitely not as elegant as it could be, but you can see here that it takes three separate operations to approximate the power of just one PCRE.

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