unicode : include '~' in collapsed symbol class (#26972)

The collapsed \p{S} class was missing '~', which split " ~" into
separate pre-tokens and prevented the Ġ~ BPE merge used by DeepSeek V4.
This caused re-tokenized prompts to diverge from sampled tokens and
broke KV cache reuse.

Assisted-by: Codex

src/unicode.cpp

@@ -1241,7 +1241,7 @@ std::vector<std::string> unicode_regex_split(const std::string & text, const std
         { unicode_cpt_flags::LETTER,      "\x41-\x5A\x61-\x7A" }, // A-Za-z
         { unicode_cpt_flags::PUNCTUATION, "\x21-\x23\x25-\x2A\x2C-\x2F\x3A-\x3B\x3F-\x40\\\x5B-\\\x5D\x5F\\\x7B\\\x7D" }, // !-#%-*,-/:-;?-@\[-\]_\{\}
         { unicode_cpt_flags::ACCENT_MARK, "" }, // no sub-128 codepoints
-        { unicode_cpt_flags::SYMBOL,      "\\\x24\\\x2B\x3C-\x3E\x5E\x60\\\x7C" }, // $+<=>^`|
+        { unicode_cpt_flags::SYMBOL,      "\\\x24\\\x2B\x3C-\x3E\x5E\x60\\\x7C\\\x7E" }, // $+<=>^`|~
     };
 
     // compute collapsed codepoints only if needed by at least one regex

tests/CMakeLists.txt

@@ -116,6 +116,8 @@ function(llama_build_and_test source)
     set_property(TEST ${TEST_TARGET} PROPERTY LABELS ${LLAMA_TEST_LABEL})
 endfunction()
 
+llama_build_and_test(test-unicode.cpp)
+
 # build test-tokenizer-0 target once and add many tests
 llama_build(test-tokenizer-0.cpp)
 

tests/test-unicode.cpp

@@ -0,0 +1,24 @@
+#include "../src/unicode.h"
+
+#include <cstdio>
+#include <string>
+#include <vector>
+
+int main() {
+    const std::vector<std::string> regex_exprs = {
+        "[~][A-Za-z]+| ?[\\p{S}]+|\\s+",
+    };
+    const std::vector<std::string> expected = { " ~", "foo" };
+    const auto actual = unicode_regex_split(" ~foo", regex_exprs, false);
+
+    if (actual != expected) {
+        fprintf(stderr, "unexpected split:");
+        for (const auto & piece : actual) {
+            fprintf(stderr, " [%s]", piece.c_str());
+        }
+        fprintf(stderr, "\n");
+        return 1;
+    }
+
+    return 0;
+}