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.

stdlib/Dates/src/io.jl

@@ -332,6 +332,21 @@ 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. So instead we
+# construct and cache it outside the method body. Note, however, that when
+# `keys(CONVERSION_SPECIFIERS)` changes, the cached Regex must be updated
+# accordingly; hence the Ref-ness of the cache, the helper method with which
+# to populate the cache, and cache of the hash of `keys(CONVERSION_SPECIFIERS)`
+# to facilitate checking for changes.
+function compute_dateformat_regex(conversion_specifiers)
+    letters = String(collect(keys(conversion_specifiers)))
+    return Regex("(?<!\\\\)([\\Q$letters\\E])\\1*")
+end
+const DATEFORMAT_REGEX_CACHE = Ref(compute_dateformat_regex(CONVERSION_SPECIFIERS))
+const CONVERSION_SPECIFIERS_KEYS_HASH = Ref(hash(keys(CONVERSION_SPECIFIERS)))
+
 """
     DateFormat(format::AbstractString, locale="english") -> DateFormat
 
@@ -379,8 +394,15 @@ 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)
+    # To understand this block, please see the comments attached to the
+    # definitions of DATEFORMAT_REGEX_CACHE and CONVERSION_SPECIFIERS_KEYS_HASH above.
+    conversion_specifiers_keys_hash = hash(keys(CONVERSION_SPECIFIERS))
+    if conversion_specifiers_keys_hash != CONVERSION_SPECIFIERS_KEYS_HASH[]
+        DATEFORMAT_REGEX_CACHE[] = compute_dateformat_regex(CONVERSION_SPECIFIERS)
+        CONVERSION_SPECIFIERS_KEYS_HASH[] = conversion_specifiers_keys_hash
+    end
+
+    for m in eachmatch(DATEFORMAT_REGEX_CACHE[], f)
         tran = replace(f[prev_offset:prevind(f, m.offset)], r"\\(.)" => s"\1")
 
         if !isempty(prev)