PHP Regular Expressions

A regular expression (regex) is a pattern used to match, search, and manipulate strings. Regular expressions are powerful tools for validating email addresses, phone numbers, and postal codes; searching text for patterns; extracting specific parts of strings; and replacing text based on patterns. PHP uses PCRE (Perl Compatible Regular Expressions) through functions prefixed with preg_.

Regular Expression Syntax

A regex pattern in PHP is written as a string enclosed in delimiters — typically forward slashes: /pattern/flags.

preg_match() — Test if a Pattern Matches

Returns 1 if the pattern is found, 0 if not. An optional third argument captures matched groups.

<?php
  $email = "user@example.com";

  if (preg_match("/^[\w.-]+@[\w.-]+\.[a-z]{2,}$/i", $email)) {
    echo "Valid email address.";
  } else {
    echo "Invalid email address.";
  }
?>

Using Capture Groups

<?php
  $dateString = "Today is 2026-03-17.";

  if (preg_match("/(\d{4})-(\d{2})-(\d{2})/", $dateString, $matches)) {
    echo "Full match: " . $matches[0];   // 2026-03-17
    echo "Year: "       . $matches[1];   // 2026
    echo "Month: "      . $matches[2];   // 03
    echo "Day: "        . $matches[3];   // 17
  }
?>

preg_match_all() — Find All Matches

<?php
  $html = '<a href="https://google.com">Google</a> and <a href="https://php.net">PHP</a>';

  preg_match_all('/href="([^"]+)"/', $html, $matches);

  // $matches[0] = full matches
  // $matches[1] = first capture group contents
  foreach ($matches[1] as $url) {
    echo $url . "\n";
    // https://google.com
    // https://php.net
  }
?>

preg_replace() — Replace Matched Patterns

<?php
  $text = "My phone is 555-123-4567 and backup is 555-987-6543.";

  // Replace phone numbers with [REDACTED]
  $redacted = preg_replace("/\d{3}-\d{3}-\d{4}/", "[REDACTED]", $text);
  echo $redacted;
  // My phone is [REDACTED] and backup is [REDACTED].

  // Remove extra whitespace
  $messy = "This   has   too   many   spaces.";
  $clean = preg_replace("/\s+/", " ", $messy);
  echo $clean;   // This has too many spaces.
?>

preg_split() — Split String by Pattern

<?php
  $csv = "apple, banana,cherry , date,  elderberry";

  // Split on comma with optional surrounding whitespace
  $fruits = preg_split("/\s*,\s*/", trim($csv));

  print_r($fruits);
  // Array ( [0] => apple [1] => banana [2] => cherry [3] => date [4] => elderberry )
?>

Practical Examples

Validate a Phone Number

<?php
  function isValidPhone(string $phone): bool {
    // Matches: (555) 123-4567, 555-123-4567, 5551234567, +1-555-123-4567
    return (bool)preg_match("/^(\+1[-\s]?)?(\(?\d{3}\)?[-\s]?)\d{3}[-\s]?\d{4}$/", $phone);
  }

  var_dump(isValidPhone("555-123-4567"));      // true
  var_dump(isValidPhone("(555) 123-4567"));    // true
  var_dump(isValidPhone("+1-555-123-4567"));   // true
  var_dump(isValidPhone("12345"));             // false
?>

Extract All Hashtags from Text

<?php
  $tweet = "Loving #PHP and #coding! Best #webdev language. #php8";

  preg_match_all("/#(\w+)/i", $tweet, $matches);

  $tags = array_map("strtolower", $matches[1]);
  $uniqueTags = array_unique($tags);

  echo implode(", ", $uniqueTags);   // php, coding, webdev, php8
?>

Validate a URL

<?php
  function isValidUrl(string $url): bool {
    $pattern = "/^(https?:\/\/)?(www\.)?[\w-]+(\.[a-z]{2,})+([\/\w .-]*)*\/?$/i";
    return (bool)preg_match($pattern, $url);
  }

  var_dump(isValidUrl("https://www.example.com"));       // true
  var_dump(isValidUrl("http://example.com/path/page"));  // true
  var_dump(isValidUrl("not a url"));                     // false
?>

Regex Flags

Key Points

• PHP uses PCRE regex through functions prefixed with preg_.
• preg_match() tests whether a pattern exists and captures groups into a third argument array.
• preg_match_all() finds every occurrence of a pattern in a string.
• preg_replace() replaces matched patterns with a replacement string.
• preg_split() splits a string using a pattern as the delimiter.
• Use the i flag for case-insensitive matching.
• Capture groups () extract specific parts of a match; they are accessible as array elements after the match.