Groups and alternation

Use parentheses to group sub-patterns for quantifiers and the pipe operator to match one of several alternatives.

Two tools dramatically expand what patterns can express: grouping with parentheses (…) and alternation with the pipe |. Together they let you say "match any one of these complete phrases" or "repeat this multi-character sequence."

Grouping with `(…)`

Without parentheses, a quantifier applies to only the single atom immediately before it:

/ab+/.test("ab");    // true  — 'a' once, 'b' one or more times
/ab+/.test("ababab"); // true — 'a' once, then many b's... actually:
                     // matches "ab" at start (a then b+)

Wait — let's be precise. /ab+/ means a followed by one-or-more bs. It does not mean "ab repeated." To repeat the pair ab, wrap it in a group:

/(ab)+/.test("ab");       // true  — "ab" once
/(ab)+/.test("ababab");   // true  — "ab" three times
/(ab)+/.test("aabb");     // false — not "ab" repeated

Groups create a unit that a quantifier treats as a single atom:

/(ha)+/.test("hahaha");   // true  — "ha" repeated
/(\d{3}-)+/.test("555-666-"); // true — "NNN-" repeated twice

Capturing vs non-capturing

By default (…) is a capturing group — it records the matched text so you can retrieve it from the result. The intermediate tier covers this in depth. For now, just know that the parentheses serve two purposes: grouping (this lesson) and capturing (later).

If you only want grouping without capturing, use (?:…) — also covered in the intermediate tier.

Alternation with `|`

The pipe | means "match either the expression on the left or the one on the right":

/cat|dog/.test("I have a cat");  // true
/cat|dog/.test("I have a dog");  // true
/cat|dog/.test("I have a fish"); // false

Alternation has very low precedence — lower than concatenation. This means /cat|dog food/ means cat OR dog food, not cat food OR dog food. Groups fix that:

/(cat|dog) food/.test("cat food"); // true
/(cat|dog) food/.test("dog food"); // true
/(cat|dog) food/.test("bird food"); // false

Without the group: /cat|dog food/.test("cat") is true because the left branch cat matches on its own.

Multiple alternatives

You can chain as many alternatives as you need:

/Mon|Tue|Wed|Thu|Fri/.test("Meeting on Wed");  // true
/^(GET|POST|PUT|DELETE|PATCH)\s/.test("POST /api/users");  // true

The engine tries each alternative from left to right and returns on the first match. If you have /a|ab/ and the input is "ab", the engine matches "a" and stops — it never reaches ab. Order matters when one alternative is a prefix of another.

Alternation inside a larger pattern

Alternation inside a group participates naturally in the surrounding pattern:

// Match both "colour" and "color"
/colou?r/.test("colour");   // true — simpler with ?
/colo(u|)r/.test("colour"); // true — alternation approach
/colo(u|)r/.test("color");  // true

// Match "Mr", "Mrs", or "Ms"
/^(Mr|Mrs|Ms)\.?\s/.test("Mrs. Smith"); // true
/^(Mr|Mrs|Ms)\.?\s/.test("Dr. Smith");  // false

Groups with quantifiers — practical examples

// US phone: (555) 123-4567 or 555-123-4567
/(\(\d{3}\)|\d{3})[-\s]\d{3}-\d{4}/.test("(555) 123-4567"); // true
/(\(\d{3}\)|\d{3})[-\s]\d{3}-\d{4}/.test("555-123-4567");   // true

// Repeated domain labels: "sub.example.com"
/(\w+\.)+\w+/.test("sub.example.com"); // true
/(\w+\.)+\w+/.test("example.com");     // true

JavaScript — editable, runs in your browser

Where to go next

Next: the dot and flags — understanding . in depth, and the five flags that change how the engine interprets a pattern.

Finished reading? Mark it complete to track your progress.

Grouping with (…)