diff --git a/src/checker.cpp b/src/checker.cpp
index 4ff7d05..1fdad3a 100644
--- a/src/checker.cpp
+++ b/src/checker.cpp
@@ -52,15 +52,16 @@ int Checker::masking(const u8 *in, int ilen, u8 *out, int olen, const char *mask
 	out[oofs] = 0;
 	return oofs;
 }
-const char *Checker::filter(const char *in, int ilen, char *out, int *olen, const char *mask) {
+const char *Checker::filter(const char *in, int ilen, char *out, int *olen, const char *mask, ContextChecker checker) {
 	if (mask == nullptr) { mask = "?"; }
 	int iofs = 0;
 	int msz = strnlen(mask, MAX_FILTER_STRING);
 	int oofs = 0, tmp;
+	void *ctx;
 	const u8 *iptr = reinterpret_cast<const u8 *>(in);
 	u8 *optr = reinterpret_cast<u8 *>(out);
 	while (iofs < ilen) {
-		if (trie_.get(iptr + iofs, ilen - iofs, &tmp) != nullptr) {
+		if ((ctx = trie_.get(iptr + iofs, ilen - iofs, &tmp)) != nullptr && checker(in, ilen, iofs, tmp, ctx)) {
 			oofs += masking(iptr + iofs, tmp, optr + oofs, *olen - oofs, mask, msz);
 			iofs += tmp;
 		} else {
@@ -76,12 +77,14 @@ const char *Checker::filter(const char *in, int ilen, char *out, int *olen, cons
 	out[*olen] = 0;
 	return out;
 }
-bool Checker::should_filter(const char *in, int ilen, int *start, int *count, void **pctx) {
+bool Checker::should_filter(const char *in, int ilen, int *start, int *count, void **pctx, ContextChecker checker) {
 	int iofs = 0, tmp;
+	void *ctx;
 	const u8 *iptr = reinterpret_cast<const u8 *>(in);
 	while (iofs < ilen) {
-		if ((*pctx = trie_.get(iptr + iofs, ilen - iofs, count)) != nullptr) {
+		if ((ctx = trie_.get(iptr + iofs, ilen - iofs, count)) != nullptr && checker(in, ilen, iofs, *count, ctx)) {
 			*start = iofs;
+			*pctx = ctx;
 			return true;
 		} else {
 			tmp = utf8::peek(iptr + iofs, ilen - iofs);
diff --git a/src/checker.h b/src/checker.h
index a3a8c9f..4e1e196 100644
--- a/src/checker.h
+++ b/src/checker.h
@@ -10,6 +10,7 @@ class WordChecker;
 class Checker {
 public:
 	typedef shutup_allocator Allocator;
+	typedef std::function<bool(const char *,int,int,int,void *)> ContextChecker; 
 	static const int MAX_FILTER_STRING = 1024 * 1024;//1M
 	class Mempool : public IMempool {
 		Allocator alloc_;
@@ -42,8 +43,9 @@ class Checker {
 	void ignore_glyphs(const char *glyphs);
 	inline void add_word(const char *s, void *ctx) { trie_.add(s, ctx); }
 	inline void remove(const char *s) { trie_.remove(s); }
-	const char *filter(const char *in, int ilen, char *out, int *olen, const char *mask = nullptr);
-	bool should_filter(const char *in, int ilen, int *start, int *count, void **pctx = nullptr);
+	static bool truer(const char *in, int ilen, int start, int count, void *ctx) { return true; }
+	const char *filter(const char *in, int ilen, char *out, int *olen, const char *mask = nullptr, ContextChecker checker = truer);
+	bool should_filter(const char *in, int ilen, int *start, int *count, void **pctx = nullptr, ContextChecker checker = truer);
 public:
 	static language::WordChecker *by(const char *lang, Mempool &m);
 	template <class C> static C *new_word_checker(Mempool &m) {
diff --git a/src/shutup.cpp b/src/shutup.cpp
index cf51a9a..cac65c9 100644
--- a/src/shutup.cpp
+++ b/src/shutup.cpp
@@ -28,14 +28,15 @@ void shutup_add_word(shutter s, const char *word, void *ctx) {
 		c->add(word, ctx);
 	}
 }
-void *shutup_should_filter(shutter s, const char *in, int ilen, int *start, int *count) {
+void *shutup_should_filter(shutter s, const char *in, int ilen, int *start, int *count, 
+	shutup_context_checker checker) {
 	void *p;
 	shutup::Checker *c = reinterpret_cast<shutup::Checker *>(s);
 	if (!c->valid()) {
 		*start = -1;
 		return nullptr;
 	}
-	if (c->should_filter(in, ilen, start, count, &p)) {
+	if (c->should_filter(in, ilen, start, count, &p, checker)) {
 		//shutup_log("should filter: length: %d [%s]\n", *olen, out);
 		return p;
 	}
@@ -54,7 +55,8 @@ static char *allocate(int *size) {
 	*size = s_buff_size;
 	return s_buff;
 }
-const char *shutup_filter(shutter s, const char *in, int ilen, char *out, int *olen, const char *mask) {
+const char *shutup_filter(shutter s, const char *in, int ilen, char *out, int *olen, const char *mask, 
+	shutup_context_checker checker) {
 	int tmp;
 	shutup::Checker *c = reinterpret_cast<shutup::Checker *>(s);
 	if (!c->valid()) {
@@ -69,7 +71,7 @@ const char *shutup_filter(shutter s, const char *in, int ilen, char *out, int *o
 			return nullptr;
 		}
 	}
-	if (c->filter(in, ilen, out, olen, mask) != nullptr) {
+	if (c->filter(in, ilen, out, olen, mask, checker) != nullptr) {
 		out[*olen] = 0;
 		return reinterpret_cast<const char *>(out);
 	}
diff --git a/src/shutup.h b/src/shutup.h
index 0d40f6b..b1d74e3 100644
--- a/src/shutup.h
+++ b/src/shutup.h
@@ -9,13 +9,16 @@ extern "C" {
 		void (*free)(void *);
 		void *(*realloc)(void *, size_t);
 	} shutup_allocator;
+	typedef bool (*shutup_context_checker)(const char *in, int ilen, int start, int count, void *ctx);
 	extern shutter shutup_new(const char *lang, shutup_allocator *a);
 	extern void shutup_delete(shutter s);
 	extern void shutup_set_alias(shutter s, const char *target, const char *alias);
 	extern void shutup_ignore_glyphs(shutter s, const char *glyphs);
 	extern void shutup_add_word(shutter s, const char *word, void *ctx);
-	extern void *shutup_should_filter(shutter s, const char *in, int ilen, int *start, int *count);
-	extern const char *shutup_filter(shutter s, const char *in, int ilen, char *out, int *olen, const char *mask);
+	extern void *shutup_should_filter(shutter s, const char *in, int ilen, int *start, int *count, 
+		shutup_context_checker checker);
+	extern const char *shutup_filter(shutter s, const char *in, int ilen, char *out, int *olen, const char *mask, 
+		shutup_context_checker checker);
 	//for debugging
 	typedef void (*shutup_logger)(const char *);
 	extern void shutup_set_logger(shutup_logger logger);
diff --git a/test/checker.cpp b/test/checker.cpp
index 15712b2..008c737 100644
--- a/test/checker.cpp
+++ b/test/checker.cpp
@@ -14,6 +14,7 @@ struct testcase {
 		const char *matched_;
 		const char *expect_;
 		void *ctx_;
+		bool (*checker_)(const char *in, int ilen, int start, int count, void *ctx);
 	};
 	struct filter {
 		const char *text_;
@@ -36,9 +37,10 @@ struct testcase {
 		}
 		for (auto &i : inputs_) {
 			int ilen = std::strlen(i.text_);
+			auto checker = (i.checker_ == nullptr ? shutup::Checker::truer : i.checker_); 
 			int start, count;
 			void *ctx;
-			if (i.filtered_ != c.should_filter(i.text_, std::strlen(i.text_), &start, &count, &ctx)) {
+			if (i.filtered_ != c.should_filter(i.text_, std::strlen(i.text_), &start, &count, &ctx, checker)) {
 				TRACE("input:[%s]\n", i.text_);
 				return "text should be filtered but actually not";
 			}
@@ -47,15 +49,15 @@ struct testcase {
 				char buff[count + 1];
 				std::memcpy(buff, i.text_ + start, count); buff[count] = 0;
 				if (std::strcmp(i.matched_, buff) != 0 || ctx != i.ctx_) {
-					TRACE("filtered:[%s] [%s]\n", i.matched_, buff);
+					TRACE("filtered:[%s] [%s] %p %p\n", i.matched_, buff, ctx, i.ctx_);
 					return "filtered but match part does not match expected";
 				}
 			}
 			int olen = ilen * shutup::utf8::MAX_BYTE_PER_GRYPH;
 			char buff[olen];
 			const char *r = mask_ == nullptr ? 
-				c.filter(i.text_, std::strlen(i.text_), buff, &olen) : 
-				c.filter(i.text_, std::strlen(i.text_), buff, &olen, mask_);
+				c.filter(i.text_, std::strlen(i.text_), buff, &olen, nullptr, checker) : 
+				c.filter(i.text_, std::strlen(i.text_), buff, &olen, mask_, checker);
 			if (std::strcmp(i.expect_, r) != 0) {
 				TRACE("filter:[%s] => [%s]\n", i.expect_, r);
 				return "filter result does not match expected";
@@ -70,6 +72,30 @@ struct testcase {
 static void *p(int id) {
 	return reinterpret_cast<void *>(id);
 }
+static bool p3_is_forward_match(const char *in, int ilen, int start, int count, void *ctx) {
+	if (ctx == p(3)) {
+		if (start == 0) {
+			return true;
+		}
+	}
+	return false;
+}
+static bool p3_is_backward_match(const char *in, int ilen, int start, int count, void *ctx) {
+	if (ctx == p(3)) {
+		if ((start + count) == ilen) {
+			return true;
+		}
+	}
+	return false;
+}
+static bool p3_is_exact_match(const char *in, int ilen, int start, int count, void *ctx) {
+	if (ctx == p(3)) {
+		if (start == 0 && count == ilen) {
+			return true;
+		}
+	}
+	return false;
+}
 
 extern const char *checker_test() {
 	std::vector<test::checker::testcase> cases{
@@ -102,6 +128,26 @@ extern const char *checker_test() {
 					p(1),
 				},
 				{"OK：「ンビエト」、NG：「ワソド」", true, "：「ワソド", "OK：「ンビエト」、NG：「???」", p(5)},
+
+				{"これは馬津怒輪亜怒ですか", false, "", "これは馬津怒輪亜怒ですか", nullptr, p3_is_exact_match},
+				{"これは馬津怒輪亜怒ですか", false, "", "これは馬津怒輪亜怒ですか", nullptr, p3_is_backward_match},
+				{"これは馬津怒輪亜怒ですか", false, "", "これは馬津怒輪亜怒ですか", nullptr, p3_is_forward_match},
+				{"これは馬津怒輪亜怒ですか", true, "馬津怒輪亜怒", "これは??????ですか", p(3)},
+				
+				{"馬津怒輪亜怒ですか", false, "", "馬津怒輪亜怒ですか", nullptr, p3_is_exact_match},
+				{"馬津怒輪亜怒ですか", false, "", "馬津怒輪亜怒ですか", nullptr, p3_is_backward_match},
+				{"馬津怒輪亜怒ですか", true, "馬津怒輪亜怒", "??????ですか", p(3), p3_is_forward_match},
+				{"馬津怒輪亜怒ですか", true, "馬津怒輪亜怒", "??????ですか", p(3)},
+				
+				{"これは馬津怒輪亜怒", false, "", "これは馬津怒輪亜怒", nullptr, p3_is_exact_match},
+				{"これは馬津怒輪亜怒", true, "馬津怒輪亜怒", "これは??????", p(3), p3_is_backward_match},
+				{"これは馬津怒輪亜怒", false, "", "これは馬津怒輪亜怒", nullptr, p3_is_forward_match},
+				{"これは馬津怒輪亜怒", true, "馬津怒輪亜怒", "これは??????", p(3)},
+				
+				{"馬津怒輪亜怒", true, "馬津怒輪亜怒", "??????", p(3), p3_is_exact_match},
+				{"馬津怒輪亜怒", true, "馬津怒輪亜怒", "??????", p(3), p3_is_backward_match},
+				{"馬津怒輪亜怒", true, "馬津怒輪亜怒", "??????", p(3), p3_is_forward_match},
+				{"馬津怒輪亜怒", true, "馬津怒輪亜怒", "??????", p(3)},
 			},
 		},
 	};