Regular Expressions in PHP

Master pattern matching and text manipulation in PHP using preg_match(), preg_replace(), and preg_split() with practical regex patterns.

Introduction to Regular Expressions

PHP uses Perl-Compatible Regular Expressions (PCRE) via the preg_* family of functions. A regex is a pattern that matches strings — used for validation, search, replace, and extraction.

1. What is a Regular Expression?

Pattern-based string matching.

2. Delimiters

Any non-alphanumeric, non-whitespace character can be a delimiter.

Regex Flags (Modifiers)

Flags are placed after the closing delimiter and modify matching behaviour.

1. Common Flags

i, m, s, x modifiers.

Character Classes

Character classes let you match any character from a defined set.

1. Shorthand Classes

\d \w \s and their negations.

2. Custom Character Classes

Use [abc] and [a-z] ranges.

Quantifiers

Quantifiers control repetition of tokens in a pattern.

1. Common Quantifiers

* + ? {n,m}

2. Greedy vs Lazy

Add ? after quantifier for lazy (non-greedy) match.

Anchors & Boundaries

Anchors assert position in the string without consuming characters.

1. ^ and $ Anchors

Match start and end of string.

2. Word Boundary \b

Match at the edge of a word.

Groups & Alternation

Groups let you capture parts of a match or apply quantifiers to sequences.

1. Capturing Groups

Use () to capture submatches.

2. Named Capturing Groups (PHP 5.2+)

Use (?P<name>...) for readable captures.

3. Non-Capturing Groups

Use (?:...) to group without capturing.

4. Alternation (OR)

Use | to match one of several options.

preg_* Functions

PHP provides several preg_* functions for different regex operations.

1. preg_match() — Single Match

Find first match and capture groups.

2. preg_match_all() — All Matches

Find every match in the string.

3. preg_replace() — Replace

Replace matches with a string.

4. preg_replace_callback() — Dynamic Replace

Use a callback to build replacement strings.

5. preg_split() — Split

Split a string by a regex pattern.

Lookahead & Lookbehind

Lookahead and lookbehind let you match patterns only when preceded or followed by another pattern.

1. Lookahead (?=...) and (?!...)

Assert what follows the match.

2. Lookbehind (?<=...) and (?<!...)

Assert what precedes the match.

Common Validation Patterns

Regex is the standard approach for validating emails, URLs, phone numbers, and more in PHP.

1. Email Validation

Practical email pattern.

2. Phone Number Validation

Match common phone formats.

3. Password Strength Check

Validate minimum password requirements.