fix: allow . to prefix prerelease in loose mode

BREAKING CHANGE: prerelease tag requires a `-` if it doesn't start with
a non-numeric value.  If it does start with a non-numeric value the `-`
is optional and a `.` can be used instead.  This means that `1.2.34.5`
is no longer a valid version in loose mode (previously it would split
into `1.2.3-4.5` and `1.2.3.4` is invalid (previously it would split
into `1.2.3-4`.

Allowing 1.2.3.4 is confusing and weird.

However, 1.2.3.beta should be allowed.

internal/re.js

@@ -58,8 +58,13 @@ createToken('PRERELEASEIDENTIFIERLOOSE', `(?:${src[t.NUMERICIDENTIFIERLOOSE]
 createToken('PRERELEASE', `(?:-(${src[t.PRERELEASEIDENTIFIER]
 }(?:\\.${src[t.PRERELEASEIDENTIFIER]})*))`)
 
-createToken('PRERELEASELOOSE', `(?:-?(${src[t.PRERELEASEIDENTIFIERLOOSE]
-}(?:\\.${src[t.PRERELEASEIDENTIFIERLOOSE]})*))`)
+// loose mode prereleases can start with a dot, but only if they're not numeric
+// 1.2.3.foo is allowed, 1.2.3.4 is not.
+createToken('PRERELEASELOOSE', `(?:(?:-|\\.*(?=[a-zA-Z-]))(${
+  src[t.PRERELEASEIDENTIFIERLOOSE]
+}(?:\\.${
+  src[t.PRERELEASEIDENTIFIERLOOSE]
+})*))`)
 
 // ## Build Metadata Identifier
 // Any combination of digits, letters, or hyphens.

test/fixtures/invalid-versions.js

@@ -12,4 +12,7 @@ module.exports = [
   [/a regexp/, 'regexp is not a string'],
   [/1.2.3/, 'semver-ish regexp is not a string'],
   [{ toString: () => '1.2.3' }, 'obj with a tostring is not a string'],
+  ['1.2.3.foo', 'no dot-led prerelease tag, strict'],
+  ['1.2.3.4', 'no dot-led numeric prerelease tag, strict'],
+  ['1.2.3.4', 'no dot-led numeric prerelease tag, loose', { loose: true }],
 ]

test/functions/valid.js

@@ -15,5 +15,9 @@ t.test('validate a version into a SemVer object', t => {
   t.equal(valid(s), '4.5.6', 'return the version if a SemVer obj')
   t.equal(valid('4.2.0foo', true), '4.2.0-foo', 'looseness as a boolean')
   t.equal(valid('4.2.0foo', { loose: true }), '4.2.0-foo', 'looseness as an option')
+  t.equal(valid('4.2.0.foo', { loose: true }), '4.2.0-foo', '.pre allowed in loose mode')
+  t.equal(valid('4.2.0.-foo', { loose: true }), '4.2.0--foo', '.-pre allowed in loose mode')
+  t.equal(valid('4.2.0--foo', { loose: true }), '4.2.0--foo', '--pre allowed in loose mode')
+  t.equal(valid('4.2.0--foo'), '4.2.0--foo', 'double-dash, strict')
   t.end()
 })