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Regex Basics Cheat Sheet

Learn core regex syntax for matching text patterns with character classes, quantifiers, and anchors.

What this cheat sheet is for

This regex basics reference helps you complete common developer tasks quickly and avoid repeated trial-and-error when working under deadlines.

  • When you need a reliable command or pattern during debugging and incident response.
  • When you are implementing a feature and want a known-good syntax example immediately.
  • When onboarding teammates and documenting repeatable workflows for daily use.
  • When reviewing pull requests and verifying that command usage follows best practices.
  • Validate simple input formats quickly.
  • Search logs for repeated error patterns.
  • Extract IDs and tokens from text.

Core syntax patterns

Match digits and words

/\d+/

Example:

/\w+/

Character classes

/[A-Z][a-z]+/

Example:

/[0-9A-F]{6}/

Quantifiers and anchors

Use quantifiers

/ab*/

Example:

/colou?r/

Start and end anchors

/^ERROR:/

Example:

/\.js$/

Practical validation snippets

Username pattern

/^[a-z0-9_]{3,16}$/

Example:

/^[A-Za-z][A-Za-z0-9_-]{2,20}$/

Simple date pattern

/^\d{4}-\d{2}-\d{2}$/

Example:

/^\d{2}\/\d{2}\/\d{4}$/

Common mistakes / Pitfalls

  • Assuming regex flavor is identical across JS, PCRE, and POSIX causes failures.
  • Greedy quantifiers can overmatch unless made lazy when needed.
  • For complex validation, regex alone may be insufficient without parser logic.

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Last updated: February 2026