Regular expressions describe text patterns. Understanding the building blocks helps you craft accurate expressions that are easy to debug.
. matches any character except newline, \d digits, \w word characters, \s whitespace.[abc] match specific sets; use [^abc] to negate.^ and $ ensure matches start or end at specific positions.*, +, ?, {m,n}) control repetition.g finds all matches; i ignores case.m treats input as multiline, making ^/$ match line boundaries.s lets . match newlines; u enables Unicode support.y (sticky) attempts matches starting at the last index in JavaScript engines.Thoughtful patterns are easier to maintain and less likely to cause performance surprises.
Use verbose mode ((?x)) or inline comments to explain non-obvious groups and avoid future mistakes.
Limit nested wildcards, prefer explicit quantifiers, and consider atomic/possessive groups to guard against exponential runtime.
Combine pattern checks with schema or business rules—especially for emails, URLs, and structured identifiers.
Are you testing patterns against short samples or large text streams?
const pattern = /\b\d{3}-\d{2}-\d{4}\b/g;
const matches = 'Visit 123-45-6789 or 987-65-4321'.match(pattern) ?? [];
console.log(matches);import re
pattern = re.compile(r'\b\d{3}-\d{2}-\d{4}\b')
text = 'Visit 123-45-6789 or 987-65-4321'
print(pattern.findall(text))
Flags such as g, i, m, s, and u adjust matching behavior—use them to find multiple results, ignore case, work with multiline input, or enable Unicode.
Keep quantifiers specific, avoid nested wildcards, and consider atomic or possessive groups to prevent exponential backtracking on crafted inputs.
No. Testing happens entirely in your browser so log snippets, payloads, and secrets remain private.
Use lookarounds to assert surrounding context without consuming characters—helpful for password policies, boundary checks, or format validation.
Simple regexes cover common cases, but no pattern fully implements RFC 5322. Pair regex with domain validation or verification emails for accuracy.