diff --git a/README.md b/README.md
index f1247f3..bf63326 100644
--- a/README.md
+++ b/README.md
@@ -354,6 +354,11 @@ Sleeps for `n` seconds, where `n` is an integer, fraction, or NVLLVS (treated as
 
 Returns an array of all non-overlapping matches of the regex `pattern` in `string`. Both arguments must be strings. Patterns use extended regular expression syntax. Returns an empty array if there are no matches. Raises an error if the pattern is invalid.
 
+### SVBSTITVE
+`SVBSTITVE(pattern, replacement, string)`
+
+Replaces all non-overlapping matches of the regex `pattern` in `string` with `replacement`. All three arguments must be strings. The replacement string supports backreferences (`\1`, `\2`, etc.) to captured groups. Returns the resulting string. Raises an error if the pattern is invalid.
+
 ## Modules
 Modules are additions to the base `CENTVRION` syntax. They add or change certain features. Modules are included in your code by having
 
diff --git a/centvrion/ast_nodes.py b/centvrion/ast_nodes.py
index 2f58de1..e2070aa 100644
--- a/centvrion/ast_nodes.py
+++ b/centvrion/ast_nodes.py
@@ -1290,6 +1290,17 @@ class BuiltIn(Node):
         except re.error as e:
           raise CentvrionError(f"Invalid regex: {e}")
         return vtable, ValList(matches)
+      case "SVBSTITVE":
+        pattern = params[0]
+        replacement = params[1]
+        text = params[2]
+        if not isinstance(pattern, ValStr) or not isinstance(replacement, ValStr) or not isinstance(text, ValStr):
+          raise CentvrionError("SVBSTITVE requires three strings")
+        try:
+          result = re.sub(pattern.value(), replacement.value(), text.value())
+        except re.error as e:
+          raise CentvrionError(f"Invalid regex: {e}")
+        return vtable, ValStr(result)
       case _:
         raise NotImplementedError(self.builtin)
 
diff --git a/centvrion/compiler/emit_expr.py b/centvrion/compiler/emit_expr.py
index c3a5cb0..4000619 100644
--- a/centvrion/compiler/emit_expr.py
+++ b/centvrion/compiler/emit_expr.py
@@ -300,6 +300,9 @@ def _emit_builtin(node, ctx):
     case "QVAERE":
       lines.append(f"CentValue {tmp} = cent_qvaere({param_vars[0]}, {param_vars[1]});")
 
+    case "SVBSTITVE":
+      lines.append(f"CentValue {tmp} = cent_svbstitve({param_vars[0]}, {param_vars[1]}, {param_vars[2]});")
+
     case _:
       raise NotImplementedError(node.builtin)
 
diff --git a/centvrion/compiler/runtime/cent_runtime.c b/centvrion/compiler/runtime/cent_runtime.c
index 449ba0f..e91e9dc 100644
--- a/centvrion/compiler/runtime/cent_runtime.c
+++ b/centvrion/compiler/runtime/cent_runtime.c
@@ -902,6 +902,99 @@ CentValue cent_qvaere(CentValue pattern, CentValue text) {
   return result;
 }
 
+/* Expand replacement string, substituting \1..\9 with captured groups */
+static void _expand_replacement(const char *repl, const char *subject,
+                                regmatch_t *matches, int ngroups,
+                                char **out, size_t *opos, size_t *ocap) {
+  for (const char *r = repl; *r; r++) {
+    if (*r == '\\' && r[1] >= '1' && r[1] <= '9') {
+      int g = r[1] - '0';
+      r++;
+      if (g < ngroups && matches[g].rm_so != -1) {
+        size_t glen = matches[g].rm_eo - matches[g].rm_so;
+        while (*opos + glen + 1 > *ocap) {
+          *ocap *= 2;
+          char *newbuf = cent_arena_alloc(cent_arena, *ocap);
+          memcpy(newbuf, *out, *opos);
+          *out = newbuf;
+        }
+        memcpy(*out + *opos, subject + matches[g].rm_so, glen);
+        *opos += glen;
+      }
+    } else if (*r == '\\' && r[1] == '\\') {
+      /* escaped backslash → literal \ */
+      if (*opos + 2 > *ocap) {
+        *ocap *= 2;
+        char *newbuf = cent_arena_alloc(cent_arena, *ocap);
+        memcpy(newbuf, *out, *opos);
+        *out = newbuf;
+      }
+      (*out)[(*opos)++] = '\\';
+      r++;
+    } else {
+      if (*opos + 2 > *ocap) {
+        *ocap *= 2;
+        char *newbuf = cent_arena_alloc(cent_arena, *ocap);
+        memcpy(newbuf, *out, *opos);
+        *out = newbuf;
+      }
+      (*out)[(*opos)++] = *r;
+    }
+  }
+}
+
+CentValue cent_svbstitve(CentValue pattern, CentValue replacement, CentValue text) {
+  if (pattern.type != CENT_STR || replacement.type != CENT_STR || text.type != CENT_STR)
+    cent_type_error("'SVBSTITVE' requires three strings");
+  regex_t re;
+  int rc = regcomp(&re, pattern.sval, REG_EXTENDED);
+  if (rc != 0) {
+    char errbuf[256];
+    regerror(rc, &re, errbuf, sizeof(errbuf));
+    regfree(&re);
+    cent_runtime_error(errbuf);
+  }
+  size_t text_len = strlen(text.sval);
+  size_t repl_len = strlen(replacement.sval);
+  size_t cap = text_len + repl_len * 4 + 1;
+  char *result = cent_arena_alloc(cent_arena, cap);
+  size_t rpos = 0;
+  const char *cursor = text.sval;
+  int ngroups = (int)re.re_nsub + 1;
+  if (ngroups > 10) ngroups = 10;
+  regmatch_t matches[10];
+  while (*cursor && regexec(&re, cursor, ngroups, matches, 0) == 0) {
+    /* copy text before match */
+    size_t prefix_len = matches[0].rm_so;
+    while (rpos + prefix_len + 1 > cap) {
+      cap *= 2;
+      char *newbuf = cent_arena_alloc(cent_arena, cap);
+      memcpy(newbuf, result, rpos);
+      result = newbuf;
+    }
+    memcpy(result + rpos, cursor, prefix_len);
+    rpos += prefix_len;
+    /* expand replacement with backreferences */
+    _expand_replacement(replacement.sval, cursor, matches, ngroups,
+                        &result, &rpos, &cap);
+    cursor += matches[0].rm_eo;
+    if (matches[0].rm_eo == 0) cursor++;
+  }
+  /* copy remaining text */
+  size_t tail_len = strlen(cursor);
+  while (rpos + tail_len + 1 > cap) {
+    cap *= 2;
+    char *newbuf = cent_arena_alloc(cent_arena, cap);
+    memcpy(newbuf, result, rpos);
+    result = newbuf;
+  }
+  memcpy(result + rpos, cursor, tail_len);
+  rpos += tail_len;
+  result[rpos] = '\0';
+  regfree(&re);
+  return cent_str(result);
+}
+
 /* ------------------------------------------------------------------ */
 /* Initialisation                                                       */
 /* ------------------------------------------------------------------ */
diff --git a/centvrion/compiler/runtime/cent_runtime.h b/centvrion/compiler/runtime/cent_runtime.h
index 4565c35..94e93b8 100644
--- a/centvrion/compiler/runtime/cent_runtime.h
+++ b/centvrion/compiler/runtime/cent_runtime.h
@@ -233,6 +233,7 @@ CentValue cent_lege(CentValue path);                            /* LEGE */
 void      cent_scribe(CentValue path, CentValue content);       /* SCRIBE */
 void      cent_adivnge(CentValue path, CentValue content);      /* ADIVNGE */
 CentValue cent_qvaere(CentValue pattern, CentValue text);       /* QVAERE */
+CentValue cent_svbstitve(CentValue pattern, CentValue replacement, CentValue text); /* SVBSTITVE */
 
 /* ------------------------------------------------------------------ */
 /* Array helpers                                                        */
diff --git a/centvrion/lexer.py b/centvrion/lexer.py
index a2c0a9b..5e65a23 100644
--- a/centvrion/lexer.py
+++ b/centvrion/lexer.py
@@ -58,7 +58,8 @@ builtin_tokens = [("BUILTIN", i) for i in [
   "LEGE",
   "SCRIBE",
   "ADIVNGE",
-  "QVAERE"
+  "QVAERE",
+  "SVBSTITVE"
 ]]
 
 data_tokens = [
diff --git a/snippets/syntaxes/centvrion.sublime-syntax b/snippets/syntaxes/centvrion.sublime-syntax
index de4a352..94d3a6f 100644
--- a/snippets/syntaxes/centvrion.sublime-syntax
+++ b/snippets/syntaxes/centvrion.sublime-syntax
@@ -70,7 +70,7 @@ contexts:
       scope: constant.language.centvrion
 
   builtins:
-    - match: '\b(ADIVNGE|AVDI_NVMERVS|AVDI|CLAVES|DECIMATIO|DIC|EVERRE|FORTVITVS_NVMERVS|FORTVITA_ELECTIO|LEGE|LONGITVDO|ORDINA|SCRIBE|SEMEN|SENATVS)\b'
+    - match: '\b(ADIVNGE|AVDI_NVMERVS|AVDI|CLAVES|DECIMATIO|DIC|EVERRE|FORTVITVS_NVMERVS|FORTVITA_ELECTIO|LEGE|LONGITVDO|ORDINA|QVAERE|SCRIBE|SEMEN|SENATVS|SVBSTITVE)\b'
       scope: support.function.builtin.centvrion
 
   modules:
diff --git a/tests.py b/tests.py
index 8c564f6..5e2cd3c 100644
--- a/tests.py
+++ b/tests.py
@@ -620,6 +620,22 @@ builtin_tests = [
   ('QVAERE("", "ab")', Program([], [ExpressionStatement(BuiltIn("QVAERE", [String(""), String("ab")]))]), ValList([ValStr(""), ValStr(""), ValStr("")])),
   # QVAERE: dot matches any character
   ('QVAERE(".", "ab")', Program([], [ExpressionStatement(BuiltIn("QVAERE", [String("."), String("ab")]))]), ValList([ValStr("a"), ValStr("b")])),
+  # SVBSTITVE: basic literal replacement
+  ('SVBSTITVE("a", "b", "aaa")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("a"), String("b"), String("aaa")]))]), ValStr("bbb")),
+  # SVBSTITVE: regex character class
+  ('SVBSTITVE("[0-9]+", "N", "abc123def456")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("[0-9]+"), String("N"), String("abc123def456")]))]), ValStr("abcNdefN")),
+  # SVBSTITVE: no match → string unchanged
+  ('SVBSTITVE("x", "y", "abc")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("x"), String("y"), String("abc")]))]), ValStr("abc")),
+  # SVBSTITVE: empty replacement (deletion)
+  ('SVBSTITVE("a", "", "banana")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("a"), String(""), String("banana")]))]), ValStr("bnn")),
+  # SVBSTITVE: empty text → empty string
+  ('SVBSTITVE("a", "b", "")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("a"), String("b"), String("")]))]), ValStr("")),
+  # SVBSTITVE: dot matches any character
+  ('SVBSTITVE(".", "x", "ab")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("."), String("x"), String("ab")]))]), ValStr("xx")),
+  # SVBSTITVE: backreference swaps two groups
+  ('SVBSTITVE("(a)(b)", "\\2\\1", "ab")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("(a)(b)"), String("\\2\\1"), String("ab")]))]), ValStr("ba")),
+  # SVBSTITVE: backreference with unmatched group (ignored)
+  ('SVBSTITVE("(a)(b)?", "\\1\\2", "a")', Program([], [ExpressionStatement(BuiltIn("SVBSTITVE", [String("(a)(b)?"), String("\\1\\2"), String("a")]))]), ValStr("a")),
 ]
 
 class TestBuiltins(unittest.TestCase):
@@ -704,6 +720,10 @@ error_tests = [
   ('QVAERE(I, "abc")', CentvrionError),                                     # QVAERE requires strings, not int
   ('QVAERE("abc", I)', CentvrionError),                                     # QVAERE requires strings, not int
   ('QVAERE("[", "abc")', CentvrionError),                                   # QVAERE invalid regex
+  ('SVBSTITVE(I, "b", "c")', CentvrionError),                                # SVBSTITVE requires strings, not int pattern
+  ('SVBSTITVE("a", I, "c")', CentvrionError),                                # SVBSTITVE requires strings, not int replacement
+  ('SVBSTITVE("a", "b", I)', CentvrionError),                                # SVBSTITVE requires strings, not int text
+  ('SVBSTITVE("[", "b", "c")', CentvrionError),                              # SVBSTITVE invalid regex
 ]
 
 class TestErrors(unittest.TestCase):