Lift expensive Regex construction from DateFormat method body.

Constructing the Regex touched in this commit can represent a
significant fraction (e.g. half or better) of the runtime of
the DateFormat method touched in this commit. To make this
DateFormat method more efficient, let's lift that Regex
construction out of that method body.

Author: Sacha0

stdlib/Dates/src/io.jl

@@ -332,6 +332,12 @@ const CONVERSION_TRANSLATIONS = IdDict{Type, Any}(
     Time => (Hour, Minute, Second, Millisecond, Microsecond, Nanosecond, AMPM),
 )
 
+# The `DateFormat(format, locale)` method just below consumes the following Regex.
+# Constructing this Regex is fairly expensive; doing so in the method itself can
+# consume half or better of `DateFormat(format, locale)`'s runtime.
+const DATEFORMAT_LETTERS = String(collect(keys(CONVERSION_SPECIFIERS)))
+const DATEFORMAT_REGEX = Regex("(?<!\\\\)([\\Q$DATEFORMAT_LETTERS\\E])\\1*")
+
 """
     DateFormat(format::AbstractString, locale="english") -> DateFormat
 
@@ -379,8 +385,7 @@ function DateFormat(f::AbstractString, locale::DateLocale=ENGLISH)
     prev = ()
     prev_offset = 1
 
-    letters = String(collect(keys(CONVERSION_SPECIFIERS)))
-    for m in eachmatch(Regex("(?<!\\\\)([\\Q$letters\\E])\\1*"), f)
+    for m in eachmatch(DATEFORMAT_REGEX, f)
         tran = replace(f[prev_offset:prevind(f, m.offset)], r"\\(.)" => s"\1")
 
         if !isempty(prev)