YAML Validator

What the YAML Validator Does

This tool scans YAML line by line and looks for the colon that separates a key from its value, reporting any non-blank, non-comment line that is missing one. It is aimed at the simplest and most common YAML mistake: forgetting the colon on a key-value line.

It is a fast sanity check for straightforward configuration files — a quick look to catch an obvious slip before you load the file into an application.

How It Works

The tool goes through each line, skips blank lines and comment lines starting with a hash, and checks the rest for a colon. If a content line has no colon, it reports the line number as a problem; otherwise it considers the input to pass.

That is the whole check. It does not build a YAML document or interpret the structure — it simply looks for the key-value separator on each line. This makes it fast, but it means it only catches one specific kind of error and is blind to everything else YAML cares about.

What it checks & how

1. Read line by line. Go through the YAML one line at a time.
2. Skip blanks and comments. Ignore empty lines and lines that start with a hash.
3. Look for a colon. Check that each remaining content line contains a colon.
4. Report missing colons. Flag any content line without a colon; otherwise pass.

What it checks

It only looks for the key-value colon. A valid list item, which uses a dash and no colon, is wrongly flagged — and real errors in indentation or structure are missed entirely.

Privacy

Your YAML never leaves your device. The check runs in your browser with no network request, so nothing is uploaded.

Because the work is local, you can check private configuration files without them being stored or sent anywhere.

Nothing persists between sessions — close the tab and the input is gone.

Standards & references

• YAML — A human-friendly data format where structure is defined by indentation, with keys and values separated by colons and lists written with dashes. Real validation requires parsing all of that, not just looking for colons.
• Indentation is significant — In YAML, indentation determines nesting and meaning. A line-by-line colon check ignores indentation completely, so it cannot catch the structural errors that are among the most common YAML problems.
• Lists use dashes, not colons — A sequence item is written with a leading dash and may have no colon at all. Because this tool requires a colon on every content line, it wrongly flags valid list entries.

Accuracy & Limitations

For its one job — catching a content line that is missing the key-value colon in simple YAML — it works, and that is a genuinely common typo worth spotting quickly.

It is not a YAML parser, so it misses almost everything a real validator checks. Wrong indentation, bad nesting, invalid types, duplicate keys, broken anchors and malformed multi-line strings all pass straight through, because the tool only looks for colons.

It produces false alarms on valid YAML. A list item written with a dash has no colon, so the tool flags it as an error even though it is perfectly correct. Flow-style and other valid constructs can be mis-flagged too.

Because of these gaps, a pass here does not mean the YAML is valid, and a flag does not always mean it is broken. Treat it as a quick check for one specific typo, and use a real parser to validate a file you rely on.

Real-World Use Cases

When to use it — and when not to

For real validation, use a proper YAML parser or linter that understands indentation, types, lists and the full spec — in your editor, a CI step, or a dedicated YAML linter. This tool only catches a missing colon on simple key-value lines.

Frequently Asked Questions

References

• YAML (Wikipedia)
• YAML 1.2 specification
• YAML Ain't Markup Language (yaml.org)

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