import YAML file, RollupError: 'Legacy octal escape is not permitted in strict mode'

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• Rollup Plugin Name: @rollup/plugin-yaml
• Rollup Plugin Version: 4.1.2
• Rollup Version: 4.60.1
• Operating System (or Browser): ubuntu 22.04
• Node Version: 24.14.0
• Link to reproduction: https://stackblitz.com/edit/rollup-repro-arqohgva

Expected Behavior

No errors occur.

Actual Behavior

[!] RollupError: src/data.yaml (1:13): Legacy octal escape is not permitted in strict mode (Note that you need plugins to import files that are not JavaScript)
src/data.yaml (1:13)
1: var data = { 01:"any value" };
                ^
2: 
3: export default data;

Additional Information

I enclose key in quotes to indicate string.

On JS-YAML online demo, converted correctly.

input

# Enclose key in quotes to indicate string

'01': any value

output

{ '01': 'any value' }

---

Is this problem with tosource, not @rollup/plugin-yaml?

Is this problem in this code?

/tosource/src/tosource.ts

function legalKey(key: string) {
  return (
-    /^([a-z_$][0-9a-z_$]*|[0-9]+)$/gi.test(key) && !KEYWORD_REGEXP.test(key)
+    /^([a-z_$][0-9a-z_$]*|0|[1-9][0-9]*)$/gi.test(key) && !KEYWORD_REGEXP.test(key) // Like this?
  );
}