spec: regular expressions (make-regexp/regexp-match/regexp-replace + split)

Adds 9 regexp primitives to stdlib.regexp. OCaml: SxRegexp(src,flags,Re.re)
using Re.Pcre; $&/$1 capture expansion in replace. JS: native RegExp
with SxRegexp wrapper; regexp-match returns {:match :start :end :groups}.
32 tests in test-regexp.sx, all pass on both hosts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

spec/primitives.sx

@@ -1196,3 +1196,59 @@
   :params (s fn)
   :returns "set"
   :doc "New set of results of (fn val) for each element in s.")
+
+(define-module :stdlib.regexp)
+
+(define-primitive
+  "make-regexp"
+  :params ((pattern :as string) &rest (flags :as string))
+  :returns "regexp"
+  :doc "Compile regexp from pattern string and optional flags string (\"i\" case-insensitive, \"m\" multiline, \"s\" dotall).")
+
+(define-primitive
+  "regexp?"
+  :params (v)
+  :returns "boolean"
+  :doc "True if v is a compiled regexp.")
+
+(define-primitive
+  "regexp-source"
+  :params ((re :as regexp))
+  :returns "string"
+  :doc "Pattern string of a regexp.")
+
+(define-primitive
+  "regexp-flags"
+  :params ((re :as regexp))
+  :returns "string"
+  :doc "Flags string of a regexp.")
+
+(define-primitive
+  "regexp-match"
+  :params ((re :as regexp) (str :as string))
+  :returns "any"
+  :doc "First match of re in str. Returns {:match \"...\" :start N :end N :groups (...)} or nil.")
+
+(define-primitive
+  "regexp-match-all"
+  :params ((re :as regexp) (str :as string))
+  :returns "list"
+  :doc "All non-overlapping matches of re in str as a list of match dicts.")
+
+(define-primitive
+  "regexp-replace"
+  :params ((re :as regexp) (str :as string) (replacement :as string))
+  :returns "string"
+  :doc "Replace first match of re in str with replacement. $& = whole match, $1..$9 = groups.")
+
+(define-primitive
+  "regexp-replace-all"
+  :params ((re :as regexp) (str :as string) (replacement :as string))
+  :returns "string"
+  :doc "Replace all matches of re in str with replacement.")
+
+(define-primitive
+  "regexp-split"
+  :params ((re :as regexp) (str :as string))
+  :returns "list"
+  :doc "Split str on every match of re; returns list of strings.")