BinaryPlatforms: prevent creating an identical regex every time a `Pl…

…atform` is parsed (#52829)

Pkg likes to call `HostPlatform()` a bit too often for its own good
which shows up in some profiles. This makes `HostPlatform()` go from 250
us to 150 us with a quite trivial code change so should be fairly
uncontroversial. (Pkg should probably also be fixed, alt. `HostPlatform`
should be cached in Base.)

base/binaryplatforms.jl

@@ -661,18 +661,12 @@ const libstdcxx_version_mapping = Dict{String,String}(
     "libstdcxx" => "-libstdcxx\\d+",
 )
 
-"""
-    parse(::Type{Platform}, triplet::AbstractString)
-
-Parses a string platform triplet back into a `Platform` object.
-"""
-function Base.parse(::Type{Platform}, triplet::String; validate_strict::Bool = false)
+const triplet_regex = let
     # Helper function to collapse dictionary of mappings down into a regex of
     # named capture groups joined by "|" operators
     c(mapping) = string("(",join(["(?<$k>$v)" for (k, v) in mapping], "|"), ")")
 
-    # We're going to build a mondo regex here to parse everything:
-    triplet_regex = Regex(string(
+    Regex(string(
         "^",
         # First, the core triplet; arch/os/libc/call_abi
         c(arch_mapping),
@@ -687,7 +681,14 @@ function Base.parse(::Type{Platform}, triplet::String; validate_strict::Bool = f
         "(?<tags>(?:-[^-]+\\+[^-]+)*)?",
         "\$",
     ))
+end
 
+"""
+    parse(::Type{Platform}, triplet::AbstractString)
+
+Parses a string platform triplet back into a `Platform` object.
+"""
+function Base.parse(::Type{Platform}, triplet::String; validate_strict::Bool = false)
     m = match(triplet_regex, triplet)
     if m !== nothing
         # Helper function to find the single named field within the giant regex