What is Regex? — Regular Expressions Explained

Definition

A regular expression (regex) is a sequence of characters that defines a search pattern for matching, extracting, and manipulating text. Regex patterns are supported by virtually every programming language and many text editors, command-line tools, and databases. They range from simple literal matches to complex patterns with quantifiers, character classes, groups, and lookaheads that can describe sophisticated text constraints in a compact notation.

How It Works

A regex engine reads the pattern and input string and attempts to find a match. Literal characters match themselves. Special metacharacters have specific meanings: . matches any character, * means "zero or more of the preceding", + means "one or more", ? means "optional", ^ anchors to the start of the string, $ to the end. Character classes like [a-z] match any lowercase letter, [^0-9] matches any non-digit. Quantifiers {3,5} match 3 to 5 repetitions. Groups () capture matching text. | is alternation (OR). Lookaheads (?=...) and lookbehinds (?<=...) match positions without consuming characters.

Common Use Cases

• ▸Validating email addresses, phone numbers, URLs, and other formatted input
• ▸Extracting data from text like dates, IP addresses, or structured records
• ▸Find-and-replace operations with captured groups in editors and code
• ▸Parsing log files and extracting structured fields from unstructured text
• ▸Tokenizing code and text for compilers, linters, and formatters

Example

// Email validation (simplified):
^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$

// Extract dates in YYYY-MM-DD:
(\d{4})-(\d{2})-(\d{2})
Match: "2024-03-15" → groups: ["2024","03","15"]

// Replace all whitespace with single space:
\s+  →  " "

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