RegEx Tutorial: Basics
This hands-on tutorial will walk you through Regular Expressions step by step. Each section builds on the last. Complete exercises to unlock your progress. Don’t worry about memorizing everything — focus on understanding the patterns.
Regular expressions look intimidating at first — that’s completely normal. Even experienced developers regularly look up regex syntax. The key is to break patterns into small, logical pieces. By the end of this tutorial, you’ll be able to read and write patterns that would have looked like gibberish an hour ago. If you get stuck, that means you’re learning — every programmer has been exactly where you are.
Three exercise types appear throughout:
- Build it (Parsons): drag and drop regex fragments into the correct order.
- Write it (Free): type a regex from scratch.
- Fix it (Fixer Upper): a broken regex is given — debug and repair it.
Your progress is saved in your browser automatically.
Literal Matching
The simplest regex is just the text you want to find. The pattern cat matches the exact characters c, a, t — in that order, wherever they appear. This means it matches inside words too: cat appears in “education” and “scatter”.
Key points:
- RegEx is case-sensitive by default:
catdoes not match “Cat” or “CAT”. - The engine scans left-to-right, reporting every non-overlapping match.
Character Classes
A character class [...] matches any single character listed inside the brackets. For example, [aeiou] matches any one lowercase vowel.
You can also use ranges: [a-z] matches any lowercase letter, [0-9] matches any digit, and [A-Za-z] matches any letter regardless of case.
To negate a class, place ^ right after the opening bracket: [^a-z] matches any character that is not a lowercase letter — digits, punctuation, spaces, etc.
Meta Characters
Writing out full character classes every time gets tedious. RegEx provides meta character escape sequences:
| meta character | Meaning | Equivalent Class |
|---|---|---|
\d |
Any digit | [0-9] |
\D |
Any non-digit | [^0-9] |
\w |
Any “word” character | [a-zA-Z0-9_] |
\W |
Any non-word character | [^a-zA-Z0-9_] |
\s |
Any whitespace | [ \t\n\r\f] |
\S |
Any non-whitespace | [^ \t\n\r\f] |
The dot . is a wildcard that matches any single character (except newline). Because the dot matches almost everything, it is powerful but easy to overuse. When you actually need to match a literal period, escape it: \.
Anchors
Before reading this section, try the first exercise below. Use what you already know to write a regex that matches only if the entire string is digits. You’ll discover a gap in your toolkit — that’s the point!
So far every pattern matches anywhere inside a string. Anchors constrain where a match can occur without consuming characters:
| Anchor | Meaning |
|---|---|
^ |
Start of string (or line in multiline mode) |
$ |
End of string (or line in multiline mode) |
\b |
Word boundary — the point between a “word” character (\w) and a “non-word” character (\W), or vice versa |
Anchors are critical for validation. Without them, the pattern \d+ would match the 42 inside "hello42world". Adding anchors — ^\d+$ — ensures the entire string must be digits.
Word boundaries (\b) let you match whole words. \bgo\b matches the standalone word “go” but not “goal” or “cargo”.
Quantifiers
Quantifiers control how many times the preceding element must appear:
| Quantifier | Meaning |
|---|---|
* |
Zero or more times |
+ |
One or more times |
? |
Zero or one time (optional) |
{n} |
Exactly n times |
{n,} |
n or more times |
{n,m} |
Between n and m times |
Common misconception: * vs +
Students frequently confuse these two. The key difference:
a*bmatchesb,ab,aab,aaab, … — theais optional (zero or more).a+bmatchesab,aab,aaab, … — at least oneais required.
If you want “one or more”, reach for +. If you genuinely mean “zero or more”, use *. Getting this wrong is one of the most common sources of regex bugs.
Alternation & Combining
The pipe | works like a logical OR: cat|dog matches either “cat” or “dog”. Alternation has low precedence, so gray|grey matches the full words — you don’t need parentheses for simple cases.
When you combine multiple regex features, patterns become expressive:
gr[ae]y— character class for the spelling variant.\d{2}:\d{2}— two digits, a colon, two digits (time format).^(0[1-9]|1[0-2])/(0[1-9]|[12]\d|3[01])$— a full date validator.
Start simple and add complexity only when tests demand it.
You’ve completed the basics! You now know how to match literal text, use character classes, metacharacters, anchors, quantifiers, and alternation.
Ready for more? Continue to the Advanced RegEx Tutorial to learn greedy vs. lazy matching, groups, lookaheads, and tackle integration challenges.