Modern PHP›Extra· 35 min read

Regular Expressions

Match and extract patterns in text — validate phone numbers, find data, and clean strings with pregmatch and pregreplace.

What you will learn

Understand what a regular expression is
Match and validate text with preg_match
Find, extract and replace patterns

What is a regular expression?

A regular expression (regex for short) is a tiny pattern language for describing the *shape* of text — "a 10-digit number", "something that looks like an email", "a word followed by a space". You write the pattern once, and PHP can then check whether text matches it, pull pieces out, or replace them. It is the standard tool for validation and text-parsing across every language.

A pattern is written between two slashes, like /.../ . A few building blocks cover most needs:

Piece	Matches
`\d`	Any digit 0-9
`\w`	A word character (letter, digit, underscore)
`+`	One or more of the previous piece
`{10}`	Exactly 10 of the previous piece
`^` ... `$`	Start and end of the text
`[a-z]`	Any one lowercase letter

Validating with preg_match

preg_match($pattern, $text) returns 1 if the text matches the pattern and 0 if it does not — perfect for validation. Here we check that a string is exactly a 10-digit Indian mobile number.

Validate a 10-digit number

<?php
  $phone = "9876543210";
  $pattern = "/^\d{10}$/";   // start, exactly 10 digits, end

  if (preg_match($pattern, $phone)) {
    echo "Valid phone number";
  } else {
    echo "Invalid — must be 10 digits";
  }
?>

Read the pattern /^\d{10}$/ piece by piece: the ^ anchors to the start of the text, \d{10} demands exactly ten digits, and $ anchors to the end — together they mean "nothing but ten digits, start to finish". preg_match returns 1 for "9876543210", so the if is true and we print "Valid".

Note: Output (in the browser): Valid phone number Without the ^ and $ anchors, a value like "abc9876543210xyz" would also pass, because the ten digits exist *somewhere* inside. The anchors force the whole string to be just those digits.

Extracting and replacing

Round brackets (...) in a pattern capture the part they wrap, so you can pull it out of $matches. And preg_replace swaps every match for new text.

Capture with brackets, replace with preg_replace

<?php
  // extract the year from a date string
  $date = "Published: 2026-06-13";
  preg_match("/(\d{4})-\d{2}-\d{2}/", $date, $matches);
  echo "Year: " . $matches[1];

  // replace all digits with a hash
  $card = "Card 4242 4242";
  echo "<br>" . preg_replace("/\d/", "#", $card);
?>

In the first part, the pattern matches a YYYY-MM-DD date, and the brackets around (\d{4}) capture the four-digit year. After preg_match runs, $matches[1] holds that captured year (slot [0] is always the whole match, [1] the first captured group). In the second part, preg_replace("/\d/", "#", $card) finds every single digit and replaces it with #, masking the numbers.

Note: Output (in the browser): Year: 2026 Card 4242 4242 → Card #### #### Capturing groups let you *extract* a specific slice; preg_replace lets you *transform* text wholesale. Together they handle most parsing and cleaning jobs.

Tip: Regex is powerful but easy to over-use. For emails, PHP’s filter_var($email, FILTER_VALIDATE_EMAIL) is more reliable than a hand-written pattern — reach for regex when there is no built-in tool.

Q. What does preg_match return when the text matches the pattern?

Answer: preg_match returns 1 on a match and 0 on no match (it fills a third $matches argument with the captured pieces if you pass one).

✍️ Practice

Validate that a PIN code is exactly 6 digits with preg_match.
Use preg_replace to remove all spaces from a string.

🏠 Homework

Write a function that extracts every hashtag (a word starting with #) from a sentence using a regex.