Regex Fundamentals

Beginner ⭐ 50 XP ⏱ 18 min #regex#patterns

Match text patterns with the core regex syntax used across tools and languages.

📖Theory

A regular expression describes a text pattern. The same core syntax works in grep, Python, JavaScript, and most editors. The building blocks:

Literals match themselves; metacharacters (. * + ? [] () ^ $) are special
Character classes: \d digit, \w word char, \s whitespace, [a-z] range
Quantifiers: * (0+), + (1+), ? (0 or 1), {2,4} (range)
Anchors: ^ start of line, $ end of line, \b word boundary

Build patterns incrementally and test them on real samples — a tool like regex101 shows exactly what each part matches.

🌍Real-World Example

\d{3}-\d{4}            matches 555-1234
^ERROR                 lines starting with ERROR
\.log$                 strings ending in .log
[A-Za-z0-9._%+-]+@…    the local part of an email
\bcat\b                the word "cat", not "category"

grep -E '\bERROR\b' app.log        # whole word ERROR
grep -E '^[0-9]{4}-[0-9]{2}-[0-9]{2}' log   # lines starting with a date

✍️Hands-On Exercise

Write a pattern that matches a 5-digit US ZIP code.
Match all lines that start with a timestamp like 2026-06-22.
Match a filename ending in .log or .txt.
Explain why \. is needed to match an IP address literally.

🧾Cheat Sheet▾

Token	Matches
`.`	any character
`\d \w \s`	digit / word / whitespace
`[abc]` `[^abc]`	set / negated set
`*` `+` `?`	0+, 1+, 0 or 1
`{n,m}`	between n and m
`^` `$`	line start / end
`\b`	word boundary
`a\|b`	a or b

💬Common Interview Questions▾

What does the dot . match, and how do you match a literal dot?

. matches any single character (except newline by default). Escape it as \. to match a literal period.

What's the difference between * and + ?

* matches zero or more of the preceding element; + matches one or more (so it requires at least one occurrence).

What does \b do?

It’s a word boundary anchor — \bcat\b matches the standalone word “cat” but not “category” or “scatter”.

📚Official Documentation

↗ regex101 — interactive tester

📝 My notes on this topic

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