JS Foundations #12 — Regular Expressions

Regular expressions are JavaScript's pattern-matching engine. You'll need them for path parsing, string escaping, input validation, and search-and-replace. This article covers the syntax, flags, methods, and common patterns interviewers expect you to know.

1. Two Ways to Create a Regex

Use the constructor when the pattern comes from a variable or includes user input (which must be escaped).

2. Flags

3. test() — Returns Boolean

With the g flag, test() advances lastIndex — a surprising stateful behavior:

When using g with test, reset lastIndex or avoid g if you just want a boolean.

4. exec() — Returns Match Details

With the g flag, exec() advances lastIndex — call it repeatedly to get all matches.

5. String Methods That Accept Regex

match(regex) — Find Matches

matchAll(regex) — All Matches with Details

replace(regex, replacement) — Replace Matches

Interview use: Path notation conversion (#16 — [0] → .0), JSON string escaping (#17).

search(regex) — Index of First Match

split(regex) — Split by Pattern

Interview use: Parsing dot/bracket paths (#6), parsing multi-class strings (#24).

6. Character Classes

7. Quantifiers

8. Anchors and Boundaries

9. Groups and Alternation

10. Lookahead and Lookbehind

11. Common Patterns

Bracket Notation Parsing (#6, #16)

JSON String Escaping (#17)

Email Validation (Basic)

Quick Reference

Interview Tips

• Use literal syntax for static patterns — /pattern/flags is cleaner and more performant.
• Use constructor for dynamic patterns — new RegExp(variable, 'g').
• Remember that g flag makes test and exec stateful — either avoid g for simple checks or reset lastIndex.
• Use matchAll instead of exec loops — cleaner for iterating all matches with groups.

Related Articles

• #6 — Implement _.get()
• #17 — Implement JSON.stringify()
• String Methods Reference