From 6f1d851e94b7fb296b538293546c685dc4ee8345 Mon Sep 17 00:00:00 2001
From: Andrew Johnson <andrew.johnson@arjohnsonau.com>
Date: Mon, 24 Aug 2026 21:14:07 +0800
Subject: [PATCH 1/3] Replace boost/regex with std locale

---
 src/stan/io/json/json_data_handler.hpp | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/src/stan/io/json/json_data_handler.hpp b/src/stan/io/json/json_data_handler.hpp
index 3e0adeb279d..226ffdde074 100644
--- a/src/stan/io/json/json_data_handler.hpp
+++ b/src/stan/io/json/json_data_handler.hpp
@@ -5,8 +5,8 @@
 #include <stan/io/json/json_handler.hpp>
 #include <stan/io/json/rapidjson_parser.hpp>
 #include <stan/io/var_context.hpp>
-#include <cctype>
 #include <iostream>
+#include <locale>
 #include <ostream>
 #include <limits>
 #include <map>
@@ -16,7 +16,6 @@
 #include <utility>
 #include <vector>
 #include <boost/algorithm/string.hpp>
-#include <boost/regex.hpp>
 
 namespace stan {
 
@@ -172,8 +171,13 @@ class json_data_handler : public stan::json::json_handler {
    *  and contain only letters, numbers, or an underscore.
    */
   bool valid_varname(const std::string& name) {
-    static const boost::regex re("[a-zA-Z][a-zA-Z0-9_]*");
-    return boost::regex_match(name, re);
+    static const std::locale locale;
+    if (name.empty() || !std::isalpha(name[0], locale)) {
+      return false;
+    }
+    return std::all_of(name.cbegin() + 1, name.cend(), [](const char c) {
+      return std::isalnum(c, locale) || c == '_';
+    });
   }
 
   bool is_array_tuples(const std::vector<std::string>& keys) {

From f6b7ec087a77d711b25bd933218b9e1ea4a16e51 Mon Sep 17 00:00:00 2001
From: Andrew Johnson <andrew.johnson@arjohnsonau.com>
Date: Mon, 24 Aug 2026 21:55:48 +0800
Subject: [PATCH 2/3] Default to C locale for varname val

---
 src/stan/io/json/json_data_handler.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/stan/io/json/json_data_handler.hpp b/src/stan/io/json/json_data_handler.hpp
index 226ffdde074..0dcc4d813b5 100644
--- a/src/stan/io/json/json_data_handler.hpp
+++ b/src/stan/io/json/json_data_handler.hpp
@@ -171,7 +171,7 @@ class json_data_handler : public stan::json::json_handler {
    *  and contain only letters, numbers, or an underscore.
    */
   bool valid_varname(const std::string& name) {
-    static const std::locale locale;
+    static const std::locale locale = std::locale::classic();
     if (name.empty() || !std::isalpha(name[0], locale)) {
       return false;
     }

From f6b5fbf7629435347dfc5a145c61c69389961da0 Mon Sep 17 00:00:00 2001
From: Andrew Johnson <andrew.johnson@arjohnsonau.com>
Date: Mon, 24 Aug 2026 22:16:37 +0800
Subject: [PATCH 3/3] Update src/stan/io/json/json_data_handler.hpp

Co-authored-by: Brian Ward <bward@flatironinstitute.org>
---
 src/stan/io/json/json_data_handler.hpp | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/stan/io/json/json_data_handler.hpp b/src/stan/io/json/json_data_handler.hpp
index 0dcc4d813b5..b2b3faba655 100644
--- a/src/stan/io/json/json_data_handler.hpp
+++ b/src/stan/io/json/json_data_handler.hpp
@@ -171,7 +171,7 @@ class json_data_handler : public stan::json::json_handler {
    *  and contain only letters, numbers, or an underscore.
    */
   bool valid_varname(const std::string& name) {
-    static const std::locale locale = std::locale::classic();
+    static const std::locale& locale = std::locale::classic();
     if (name.empty() || !std::isalpha(name[0], locale)) {
       return false;
     }