(#42139) Fixes _is_mailto in resolution of autolink in Markdown module (#42140)

(cherry picked from commit 47797a1)

Author: tlienart

stdlib/Markdown/src/Common/inline.jl

@@ -146,13 +146,10 @@ function _is_link(s::AbstractString)
 end
 
 # non-normative regex from the HTML5 spec
-const _email_regex = r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"
+const _email_regex = r"^mailto\:[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$"
 
 function _is_mailto(s::AbstractString)
-    length(s) < 6 && return false
-    # slicing strings is a bit risky, but this equality check is safe
-    lowercase(s[1:6]) == "mailto:" || return false
-    return occursin(_email_regex, s[6:end])
+    return occursin(_email_regex, s)
 end
 
 # –––––––––––

stdlib/Markdown/test/runtests.jl

@@ -1230,3 +1230,11 @@ end
     @test sprint(show, MIME("text/plain"), s) == "  Misc:\n  - line"
 end
 
+@testset "issue #42139: autolink" begin
+    # ok
+    @test md"<mailto:[email protected]>" |> html == """<p><a href="mailto:[email protected]">mailto:[email protected]</a></p>\n"""
+    # not ok
+    @test md"<mailto [email protected]>" |> html == """<p>&lt;mailto [email protected]&gt;</p>\n"""
+    # see issue #42139
+    @test md"<一轮红日初升>" |> html == """<p>&lt;一轮红日初升&gt;</p>\n"""
+end