YAML Syntax Cheat Sheet

A concise reference covering YAML indentation rules, scalars, collections, anchors, and multi-document files.

1 PageBeginnerFeb 5, 2026

Scalars & Mappings

Key-value pairs and basic scalar types.

yaml

name: Alice          # string (unquoted)age: 30               # integerheight: 1.68           # floatactive: true            # booleannickname: ~              # null (also 'null' or empty)description: "Uses colon: needs quotes"multiline: |  Line one  Line twofolded: >  This becomes  one long line.

Lists & Nesting

Sequences and nested structures.

yaml

fruits:  - apple  - banana  - cherry# Inline (flow) stylecolors: [red, green, blue]person: { name: Bob, age: 25 }users:  - name: Alice    role: admin  - name: Bob    role: viewer

Anchors, Aliases & Multi-doc

Reusing values and separating multiple documents in one file.

yaml

defaults: &defaults  adapter: postgres  host: localhostdevelopment:  <<: *defaults  database: dev_dbtest:  <<: *defaults  database: test_db---# Second document starts herename: doc2

Rules & Common Gotchas

Indentation and quoting rules that trip people up.

Indentation- Uses spaces only, never tabs; consistent indent defines nesting level
Colons in strings- `key: value: extra` breaks parsing — quote the whole value: `key: "value: extra"`
Booleans- `true/false`, `yes/no`, `on/off` all parse as booleans depending on the YAML version/parser
Comments- Start with `#`; no block comment syntax exists
Document separator- `---` starts a new document, `...` optionally ends one
Quoting numbers as strings- Wrap in quotes (e.g. `version: "1.0"`) to prevent numeric coercion

Pro Tip

Watch out for the 'Norway problem': an unquoted `no` or `yes` value can be silently parsed as a boolean by YAML 1.1 parsers — always quote country codes and similar short strings.

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