Regex for Beginners: How to Write Regular Expressions

Regex for Beginners

Regular expressions — regex — are patterns that describe text. With regex, you can search strings, validate input, extract data, and perform complex find-and-replace using a single compact expression. Once you understand the fundamentals, regex becomes one of the most powerful tools in your programming toolkit.

What Is a Regular Expression?

A regex is a sequence of characters defining a search pattern. Pattern \d{3}-\d{2}-\d{4} matches a US Social Security number: three digits, hyphen, two digits, hyphen, four digits.

Regex engines exist in virtually every programming language — JavaScript, Python, Java, Ruby, Go — all with built-in support.

Literal Characters

Most characters match themselves. The pattern cat matches the literal string "cat" wherever it appears.

Pattern: cat
Matches: "cat" in "The cat sat on the mat."
Matches: "cat" in "catalog"

Literal matching is straightforward but limited. The real power comes from metacharacters.

Metacharacters

These characters have special meaning and must be escaped with \ to match literally:

. ^ $ * + ? { } [ ] \ | ( )

Character Classes

Character classes match one character from a set.

Square Bracket Notation

| Pattern | Matches | |---------|---------| | [aeiou] | Any vowel | | [0-9] | Any digit | | [a-zA-Z] | Any letter | | [^0-9] | Any character that is NOT a digit |

Predefined Classes

| Shorthand | Description | |-----------|-------------| | \d | Any digit (same as [0-9]) | | \w | Letter, digit, or underscore | | \s | Space, tab, newline | | \D | Any non-digit | | \W | Any non-word character | | \S | Any non-whitespace | | . | Any character except newline |

Quantifiers

Quantifiers control how many times the preceding element must appear.

| Quantifier | Meaning | Example | Matches | |------------|---------|---------|---------| | * | Zero or more | a* | "", "a", "aa" | | + | One or more | a+ | "a", "aa", "aaa" | | ? | Zero or one | colou?r | "color", "colour" | | {3} | Exactly N | \d{3} | "123" | | {2,4} | Between N and M | \d{2,4} | "12", "123", "1234" |

Examples

\d+     → "42" in "Answer is 42"
https?://  → "http://" and "https://"

Anchors

Anchors match positions, not characters.

| Anchor | Meaning | |--------|---------| | ^ | Start of string | | $ | End of string | | \b | Word boundary | | \B | Non-word boundary |

^hello   → matches "hello" only at the start
end$     → matches "end" only at the end
\bword\b → matches "word" as a whole word, not inside "sword"

Groups and Alternation

Capturing groups ( ) extract matched portions:

(\d{3})-(\d{4}) → "555-1234", captures "555" in group 1, "1234" in group 2

Non-capturing groups (?: ) group without extracting:

(?:https?|ftp):// → "http://", "https://", "ftp://"

Alternation | acts as OR:

cat|dog|bird → matches "cat", "dog", or "bird"

Practical Regex Examples

| Use Case | Pattern | |----------|---------| | US phone | \d{3}-\d{3}-\d{4} | | Email (simple) | \S+@\S+\.\S+ | | IP address | \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3} | | Hashtag | #[a-zA-Z]\w* | | Time (HH:MM) | [01]\d|2[0-3]:[0-5]\d |

How to Test a Regex Pattern

Always test patterns interactively:

1. Enter your regex pattern.
2. Enter a test string.
3. See which portions match in real time.
4. Adjust and repeat.

Use the regex tester tool to experiment with patterns. It highlights matches, shows capture groups, and helps you understand exactly what each part of your expression does.