JavaScript RegExp (x|y) Alternation

What Is (x|y)?

(x|y) is alternation (logical OR for patterns). The engine tries to match x; if that fails at the current position, it tries y. Parentheses group the branches so | applies to whole sub-patterns, not just one character.

/(cat|dog)/   → "cat" OR "dog"
/(https|http)/ → "https" OR "http"
/(red|green|blue)/ → any of three colors

/[xy]/        → one character: x OR y (class)
/(x|y)/       → sub-pattern x OR sub-pattern y

"I like cats".match(/(cat|dog)/)  → ["cat", "cat"]

How to Use (x|y)

List alternatives with | inside parentheses, then use test, match, or replace.

1. Write /(optionA|optionB)/ (add more branches with more |).
2. Use test() to check if either alternative appears.
3. Use match() with g to collect every hit.
4. Read capture group 1 when you need which branch matched.

Pseudocode

pattern = (x | y)

at current position in text:
    try match x
    if x succeeds → success (branch x)
    else try match y
    if y succeeds → success (branch y)
    else → fail at this position

// engine may advance and retry later in the string

Cheat Sheet

Keep these facts handy while writing OR patterns.

Live Preview

Pick a sample string and see matches of /(cat|dog)/g.

text = "I like cats"
text.match(/(cat|dog)/g)
→ ["cat"]

/* "cat" branch succeeds inside "cats" */

JavaScript Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

const pet = /(cat|dog)/;

console.log(pet.test("I like cats"));  // true
console.log(pet.test("dog park"));     // true
console.log(pet.test("bird house"));   // false

true
true
false

const text = "cat and dog";

console.log(text.match(/(cat|dog)/g));       // ["cat", "dog"]
console.log("catalog".match(/(cat|dog)/g));  // ["cat"]
console.log("bird".match(/(cat|dog)/g));     // null

cat,dog
cat
null

const text = "See cat or dog";
const m = text.match(/(cat|dog)/);

console.log(m[0]);                         // "cat"
console.log(m[1]);                         // "cat"  (group 1)
console.log(text.replace(/(cat|dog)/g, "pet")); // "See pet or pet"

cat
cat
See pet or pet

Performance Notes

Simple alternation is linear for these patterns. Prefer test for yes/no checks. Very large OR lists can get slower — keep branches short when you can.

Key Takeaways