Provide additional guarantees about web-features identifiers (#2974)

Fixes #2973.

Author: foolip

docs/guidelines.md

@@ -62,7 +62,7 @@ Feature authors should (in descending order of priority):
 
 A feature's identifier is the feature's filename before the `.yml` extension.
 
-Feature identifiers must contain only lowercase alphanumeric characters (a-z and 0-9) plus the `-` character (hyphen or minus sign) as a word separator.
+Feature identifiers must start with a lowercase alphabetic character (a-z) and contain only lowercase alphanumeric characters (a-z and 0-9) plus the `-` character (hyphen or minus sign) as a word separator.
 
 The identifier should match the name, with these additional guidelines:
 

index.ts

@@ -25,7 +25,8 @@ const descriptionMaxLength = 300;
 // of a draft directory doesn't work.
 const draft = Symbol('draft');
 
-const identifierPattern = /^[a-z0-9-]*$/;
+// This must match the definition in docs/guidelines.md
+const identifierPattern = /^[a-z][a-z0-9]*(-[a-z0-9]+)*$/;
 
 function* yamlEntries(root: string): Generator<[string, any]> {
     const filePaths = new fdir()
@@ -39,7 +40,7 @@ function* yamlEntries(root: string): Generator<[string, any]> {
         const { name: key } = path.parse(fp);
 
         if (!identifierPattern.test(key)) {
-            throw new Error(`${key} is not a valid identifier (must be lowercase a-z, 0-9, and hyphens)`);
+            throw new Error(`${key} is not a valid identifier (see guidelines)`);
         }
 
         const data = YAML.parse(fs.readFileSync(fp, { encoding: 'utf-8'}));