Regular Expressions for Beginners: A Practical Starter Guide

A regular expression, or regex, is a compact way to describe a pattern in text. With one line you can find phone numbers, check an email format, clean up spaces or rename hundreds of lines. Regex looks cryptic at first, but a small set of building blocks covers most everyday needs.

Literal characters

The simplest regex is just text. The pattern cat matches the letters c, a, t in that order wherever they appear, including inside "category".

Character classes

• . matches any single character except a line break.
• \d matches a digit, and \D matches anything that is not a digit.
• \w matches a letter, digit or underscore, and \W the opposite.
• \s matches whitespace such as spaces and tabs.
• [abc] matches one of the listed characters, [a-z] matches a range and [^abc] matches anything except those.

Quantifiers: how many times

• * means zero or more, + means one or more and ? means zero or one.
• {3} means exactly three, {2,5} means two to five and {3,} means three or more.

For example, \d{4} matches four digits in a row, such as a year.

Anchors and boundaries

• ^ matches the start of the text (or line with the multiline flag) and $ the end.
• \b matches a word boundary. \bcat\b finds the word "cat" but not "category".

Groups and alternatives

Parentheses group parts of a pattern and capture the matched text. A vertical bar means "or". The pattern (Mr|Ms|Mrs)\. (\w+) matches titles followed by a name, and captures the title and name separately so you can reuse them in a replacement.

Flags

• g finds all matches instead of stopping at the first.
• i ignores upper and lower case.
• m makes ^ and $ work on each line.

Three useful examples

• Extra spaces: \s{2,} finds two or more whitespace characters in a row, which you can replace with a single space.
• Simple date like 2026-11-08: \d{4}-\d{2}-\d{2}
• Hex color like #4f46e5: #[0-9a-fA-F]{6}\b

Greedy versus lazy

Quantifiers are greedy by default: they match as much as possible. On the text <b>one</b> and <b>two</b>, the pattern <b>.*</b> matches the entire string. Adding a question mark, as in <b>.*?</b>, makes it lazy, so it matches each tag pair separately.

Common mistakes

• Forgetting to escape special characters. To match a literal dot, write \., not ..
• Trying to validate everything with regex. A perfect email or URL pattern is extremely complicated. A simple pattern plus a confirmation step is usually better.
• Patterns that are too broad. .* is tempting but often matches more than you want.
• Slow patterns. Nested quantifiers such as (a+)+ can cause catastrophic backtracking on certain inputs.

Test as you build

The best way to learn regex is to experiment. Paste sample text into our Regex Tester, build your pattern a piece at a time and watch the matches and capture groups highlight live. Note that regex syntax differs slightly between programming languages. Our tester uses the JavaScript flavor.

Frequently asked questions