diff --git a/qse/include/qse/awk/awk.h b/qse/include/qse/awk/awk.h
index eab6718b..6b53c473 100644
--- a/qse/include/qse/awk/awk.h
+++ b/qse/include/qse/awk/awk.h
@@ -1,5 +1,5 @@
 /*
- * $Id: awk.h 307 2009-11-25 13:32:20Z hyunghwan.chung $
+ * $Id: awk.h 311 2009-12-09 11:35:54Z hyunghwan.chung $
  *
     Copyright 2006-2009 Chung, Hyung-Hwan.
     This file is part of QSE.
@@ -791,14 +791,12 @@ enum qse_awk_errnum_t
 	QSE_AWK_EREXRPAREN,    /**< a right parenthesis is expected */
 	QSE_AWK_EREXRBRACK,    /**< a right bracket is expected */
 	QSE_AWK_EREXRBRACE,    /**< a right brace is expected */
-	QSE_AWK_EREXUNBALPAREN,/**< unbalanced parenthesis */
-	QSE_AWK_EREXINVALBRACE,/**< invalid brace */
 	QSE_AWK_EREXCOLON,     /**< a colon is expected */
 	QSE_AWK_EREXCRANGE,    /**< invalid character range */
 	QSE_AWK_EREXCCLASS,    /**< invalid character class */
 	QSE_AWK_EREXBOUND,     /**< invalid occurrence bound */
+	QSE_AWK_EREXSPCAWP,    /**< special character at wrong position */
 	QSE_AWK_EREXPREEND,    /**< premature end of regular expression */
-	QSE_AWK_EREXGARBAGE,   /**< garbage after pattern */
 
 	/* the number of error numbers, internal use only */
 	QSE_AWK_NUMERRNUM 
diff --git a/qse/include/qse/cmn/lda.h b/qse/include/qse/cmn/lda.h
index 63dd50e1..e41c260e 100644
--- a/qse/include/qse/cmn/lda.h
+++ b/qse/include/qse/cmn/lda.h
@@ -1,5 +1,5 @@
 /*
- * $Id: lda.h 287 2009-09-15 10:01:02Z hyunghwan.chung $
+ * $Id: lda.h 311 2009-12-09 11:35:54Z hyunghwan.chung $
  *
     Copyright 2006-2009 Chung, Hyung-Hwan.
     This file is part of QSE.
@@ -87,7 +87,7 @@ typedef enum   qse_lda_walk_t qse_lda_walk_t;
  * SYNOPSIS
  */
 typedef void* (*qse_lda_copier_t) (
-	qse_lda_t* lda    /* a lda */,
+	qse_lda_t* lda    /* lda */,
 	void*      dptr   /* the pointer to data to copy */,
 	qse_size_t dlen   /* the length of data to copy */
 );
@@ -99,7 +99,7 @@ typedef void* (*qse_lda_copier_t) (
  * SYNOPSIS
  */
 typedef void (*qse_lda_freeer_t) (
-	qse_lda_t* lda    /* a lda */,
+	qse_lda_t* lda    /* lda */,
 	void*      dptr   /* the pointer to data to free */,
 	qse_size_t dlen   /* the length of data to free */
 );
@@ -141,7 +141,7 @@ typedef int (*qse_lda_comper_t) (
  * SYNOPSIS
  */
 typedef void (*qse_lda_keeper_t) (
-	qse_lda_t* lda     /* a lda */,
+	qse_lda_t* lda     /* lda */,
 	void* vptr         /* the pointer to a value */,
 	qse_size_t vlen    /* the length of a value */	
 );
@@ -259,72 +259,63 @@ void qse_lda_fini (
 /******/
 
 int qse_lda_getscale (
-	qse_lda_t* lda   /* a lda */
+	qse_lda_t* lda   /* lda */
 );
 
-/****f* Common/qse_lda_setscale
- * NAME
- *  qse_lda_setscale - set the scale factor
- *
- * DESCRIPTION 
- *  The qse_lda_setscale() function sets the scale factor of the length
- *  of a key and a value. A scale factor determines the actual length of
- *  a key and a value in bytes. A lda is created with a scale factor of 1.
- *  The scale factor should be larger than 0 and less than 256.
- *
- * NOTES
- *  It is a bad idea to change the scale factor when a lda is not empty.
- *  
- * SYNOPSIS
+/**
+ * The qse_lda_setscale() function sets the scale factor of the length
+ * of a key and a value. A scale factor determines the actual length of
+ * a key and a value in bytes. A lda is created with a scale factor of 1.
+ * The scale factor should be larger than 0 and less than 256.
+ * It is a bad idea to change the scale factor when @a lda is not empty.
  */
 void qse_lda_setscale (
-	qse_lda_t* lda   /* a lda */,
+	qse_lda_t* lda   /* lda */,
 	int scale        /* a scale factor */
 );
-/******/
 
 qse_lda_copier_t qse_lda_getcopier (
-	qse_lda_t* lda   /* a lda */
+	qse_lda_t* lda   /* lda */
 );
 
-/****f* Common/qse_lda_setcopier
- * NAME 
- *  qse_lda_setcopier - specify how to clone an element
- *
- * DESCRIPTION
- *  A special copier QSE_LDA_COPIER_INLINE is provided. This copier enables
- *  you to copy the data inline to the internal node. No freeer is invoked
- *  when the node is freeed.
- *
- *  You may set the copier to QSE_NULL to perform no special operation 
- *  when the data pointer is rememebered.
- *
- * SYNOPSIS
+/**
+ * The qse_lda_setcopier() specifies how to clone an element. The special 
+ * copier #QSE_LDA_COPIER_INLINE copies the data inline to the internal node.
+ * No freeer is invoked when the node is freeed. You may set the copier to 
+ * #QSE_LDA_COPIER_SIMPLE to perform no special operation when the data 
+ * pointer is stored.
  */
 void qse_lda_setcopier (
-	qse_lda_t* lda           /* a lda */, 
-	qse_lda_copier_t copier  /* an element copier */
+	qse_lda_t* lda           /** lda */, 
+	qse_lda_copier_t copier  /** element copier */
 );
-/******/
 
 qse_lda_freeer_t qse_lda_getfreeer (
-	qse_lda_t*   lda  /* a lda */
+	qse_lda_t*   lda  /**< lda */
 );
 
-/****f* Common/qse_lda_setfreeer
- * NAME 
- *  qse_lda_setfreeer - specify how to destroy an element
- *
- * DESCRIPTION
- *  The freeer is called when a node containing the element is destroyed.
- *
- * SYNOPSIS
+/**
+ * The qse_lda_setfreeer() function specifies how to destroy an element.
+ * The @a freeer is called when a node containing the element is destroyed.
  */
 void qse_lda_setfreeer (
-	qse_lda_t* lda           /* a lda */,
-	qse_lda_freeer_t freeer  /* an element freeer */
+	qse_lda_t* lda           /**< lda */,
+	qse_lda_freeer_t freeer  /**< element freeer */
+);
+
+qse_lda_comper_t qse_lda_getcomper (
+	qse_lda_t*   lda  /**< lda */
+);
+
+/**
+ * The qse_lda_setcomper() function specifies how to compare two elements
+ * for equality test. The comparator @a comper must return 0 if two elements
+ * compared are equal, or a non-zero number otherwise.
+ */
+void qse_lda_setcomper (
+	qse_lda_t*       lda     /**< lda */,
+	qse_lda_comper_t comper  /**< comparator */
 );
-/******/
 
 qse_lda_keeper_t qse_lda_getkeeper (
         qse_lda_t* lda
diff --git a/qse/include/qse/cmn/rex.h b/qse/include/qse/cmn/rex.h
index 1e3fb4d7..5d6fc788 100644
--- a/qse/include/qse/cmn/rex.h
+++ b/qse/include/qse/cmn/rex.h
@@ -1,5 +1,5 @@
 /*
- * $Id: rex.h 310 2009-12-08 13:15:00Z hyunghwan.chung $
+ * $Id: rex.h 311 2009-12-09 11:35:54Z hyunghwan.chung $
  *
     Copyright 2006-2009 Chung, Hyung-Hwan.
     This file is part of QSE.
@@ -66,12 +66,14 @@
 
 enum qse_rex_option_t
 {
-	QSE_REX_BUILD_NOBOUND    = (1 << 0),
+	QSE_REX_BUILD_NOBOUND    = (1 << 1),
 	QSE_REX_MATCH_IGNORECASE = (1 << 8),
 
+	/**< do not allow a special character at normal character position */
+	QSE_REX_STRICT  = (1 << 0),
 
 	/**< do not support the {n,m} style occurrence specifier */
-	QSE_REX_NOBOUND = (1 << 0),
+	QSE_REX_NOBOUND = (1 << 1),
 
 #if 0
 	QSE_REX_ESQ_HEX   = (1 << 1), /* \xhh and \uhhhh */
@@ -92,14 +94,12 @@ enum qse_rex_errnum_t
         QSE_REX_ERPAREN,       /**< right parenthesis expected */
         QSE_REX_ERBRACK,       /**< right bracket expected */
         QSE_REX_ERBRACE,       /**< right brace expected */
-        QSE_REX_EUNBALPAREN,   /**< unbalanced parenthesis */
-        QSE_REX_EINVALBRACE,   /**< invalid brace */
         QSE_REX_ECOLON,        /**< colon expected */
         QSE_REX_ECRANGE,       /**< invalid character range */
         QSE_REX_ECCLASS,       /**< invalid character class */
         QSE_REX_EBOUND,        /**< invalid occurrence bound */
-        QSE_REX_EPREEND,       /**< premature expression end */
-        QSE_REX_EGARBAGE       /**< garbage after expression */
+        QSE_REX_ESPCAWP,       /**< special character at wrong position */
+        QSE_REX_EPREEND        /**< premature expression end */
 };
 typedef enum qse_rex_errnum_t qse_rex_errnum_t;
 
diff --git a/qse/lib/awk/err.c b/qse/lib/awk/err.c
index c75fe0fd..951f0514 100644
--- a/qse/lib/awk/err.c
+++ b/qse/lib/awk/err.c
@@ -1,5 +1,5 @@
 /*
- * $Id: err.c 307 2009-11-25 13:32:20Z hyunghwan.chung $
+ * $Id: err.c 311 2009-12-09 11:35:54Z hyunghwan.chung $
  *
     Copyright 2006-2009 Chung, Hyung-Hwan.
     This file is part of QSE.
@@ -143,14 +143,12 @@ const qse_char_t* qse_awk_dflerrstr (qse_awk_t* awk, qse_awk_errnum_t errnum)
 		QSE_T("right parenthesis expected in regular expression"),
 		QSE_T("right bracket expected in regular expression"),
 		QSE_T("right brace expected in regular expression"),
-		QSE_T("unbalanced parenthesis in regular expression"),
-		QSE_T("invalid brace in regular expression"),
 		QSE_T("colon expected in regular expression"),
 		QSE_T("invalid character range in regular expression"),
 		QSE_T("invalid character class in regular expression"),
 		QSE_T("invalid occurrence bound in regular expression"),
-		QSE_T("premature end of regular expression"),
-		QSE_T("garbage after regular expression")
+		QSE_T("special character at wrong position"),
+		QSE_T("premature end of regular expression")
 	};
 
 	return (errnum >= 0 && errnum < QSE_COUNTOF(errstr))?
diff --git a/qse/lib/awk/misc.c b/qse/lib/awk/misc.c
index 35e941ac..0a69f9c5 100644
--- a/qse/lib/awk/misc.c
+++ b/qse/lib/awk/misc.c
@@ -1,5 +1,5 @@
 /*
- * $Id: misc.c 307 2009-11-25 13:32:20Z hyunghwan.chung $
+ * $Id: misc.c 311 2009-12-09 11:35:54Z hyunghwan.chung $
  *
     Copyright 2006-2009 Chung, Hyung-Hwan.
     This file is part of QSE.
@@ -1035,14 +1035,12 @@ qse_char_t* qse_awk_rtx_strxnfld (
 	 (err == QSE_REX_ERPAREN)?     QSE_AWK_EREXRPAREN: \
 	 (err == QSE_REX_ERBRACK)?     QSE_AWK_EREXRBRACK: \
 	 (err == QSE_REX_ERBRACE)?     QSE_AWK_EREXRBRACE: \
-	 (err == QSE_REX_EUNBALPAREN)? QSE_AWK_EREXUNBALPAREN: \
-	 (err == QSE_REX_EINVALBRACE)? QSE_AWK_EREXINVALBRACE: \
 	 (err == QSE_REX_ECOLON)?      QSE_AWK_EREXCOLON: \
 	 (err == QSE_REX_ECRANGE)?     QSE_AWK_EREXCRANGE: \
 	 (err == QSE_REX_ECCLASS)?     QSE_AWK_EREXCCLASS: \
 	 (err == QSE_REX_EBOUND)?      QSE_AWK_EREXBOUND: \
+	 (err == QSE_REX_ESPCAWP)?     QSE_AWK_EREXSPCAWP: \
 	 (err == QSE_REX_EPREEND)?     QSE_AWK_EREXPREEND: \
-	 (err == QSE_REX_EGARBAGE)?    QSE_AWK_EREXGARBAGE: \
 	                               QSE_AWK_EINTERN)
 
 void* qse_awk_buildrex (
diff --git a/qse/lib/cmn/rex.c b/qse/lib/cmn/rex.c
index fcb9a362..e5d78d68 100644
--- a/qse/lib/cmn/rex.c
+++ b/qse/lib/cmn/rex.c
@@ -1,5 +1,5 @@
 /*
- * $Id: rex.c 307 2009-11-25 13:32:20Z hyunghwan.chung $
+ * $Id: rex.c 311 2009-12-09 11:35:54Z hyunghwan.chung $
  * 
     Copyright 2006-2009 Chung, Hyung-Hwan.
     This file is part of QSE.
@@ -384,22 +384,7 @@ void* qse_buildrex (
 	if (builder.ptn.curc.type != CT_EOF)
 	{
 		if (errnum != QSE_NULL) 
-		{
-			if (builder.ptn.curc.type ==  CT_SPECIAL &&
-			    builder.ptn.curc.value == QSE_T(')'))
-			{
-				*errnum = QSE_REX_EUNBALPAREN;
-			}
-			else if (builder.ptn.curc.type ==  CT_SPECIAL &&
-			         builder.ptn.curc.value == QSE_T('{'))
-			{
-				*errnum = QSE_REX_EINVALBRACE;
-			}
-			else
-			{
-				*errnum = QSE_REX_EGARBAGE;
-			}
-		}
+			*errnum = QSE_REX_ESPCAWP;
 
 		QSE_MMGR_FREE (builder.mmgr, builder.code.buf);
 		return QSE_NULL;
diff --git a/qse/lib/cmn/rex1.c b/qse/lib/cmn/rex1.c
index 458dca71..d91b5542 100644
--- a/qse/lib/cmn/rex1.c
+++ b/qse/lib/cmn/rex1.c
@@ -210,14 +210,12 @@ const qse_char_t* qse_rex_geterrmsg (qse_rex_t* rex)
 		QSE_T("right parenthesis expected"),
 		QSE_T("right bracket expected"),
 		QSE_T("right brace expected"),
-		QSE_T("unbalanced parenthesis"),
-		QSE_T("invalid brace"),
 		QSE_T("colon expected"),
 		QSE_T("invalid character range"),
 		QSE_T("invalid character class"),
 		QSE_T("invalid occurrence bound"),
-		QSE_T("premature expression end"),
-		QSE_T("garbage after expression"),
+		QSE_T("special character at wrong position"),
+		QSE_T("premature expression end")
 	};
 	
 	return (rex->errnum >= 0 && rex->errnum < QSE_COUNTOF(errstr))?
@@ -793,6 +791,20 @@ static qse_rex_node_t* comp2 (comp_t* com)
 				break;
 
 			default:
+				if (com->rex->option & QSE_REX_STRICT)
+				{
+					qse_char_t spc[] = QSE_T(")?*+{");
+
+					if (com->rex->option & QSE_REX_NOBOUND)
+						spc[4] = QSE_T('\0');
+
+					if (qse_strchr (spc, com->c.value) != QSE_NULL)
+					{
+						com->rex->errnum = QSE_REX_ESPCAWP;
+						return QSE_NULL;
+					}
+				}
+
 				goto normal_char;
 		}
 	}
@@ -1202,13 +1214,21 @@ else
 qse_printf (QSE_T("adding %d NA\n"), node->id);
 */
 		
-if (qse_lda_search (
-	&e->cand.set[e->cand.pending],
-	0,
-	&cand, 1) != QSE_LDA_NIL)
-{
-return 0;
-}
+	if (qse_lda_search (
+		&e->cand.set[e->cand.pending],
+		0, &cand, 1) != QSE_LDA_NIL)
+	{
+		/* exclude any existing entries in the array.
+		 * see comp_cand() for the equality test used.
+		 * note this linear search may be a performance bottle neck	
+		 * if the arrary grows large. not so sure if it should be
+		 * switched to a different data structure such as a hash table.
+		 * the problem is that most practical regular expressions
+		 * won't have many candidates for a particular match point.
+		 * so i'm a bit skeptical about data struct switching.
+		 */
+		return 0;
+	}
 
 	if (qse_lda_insert (
 		&e->cand.set[e->cand.pending],
@@ -1810,8 +1830,9 @@ static int comp_cand (qse_lda_t* lda,
 {
 	cand_t* c1 = (cand_t*)dptr1;
 	cand_t* c2 = (cand_t*)dptr2;
-	if (c1->node == c2->node) return 0;
-	return 1;
+	return (c1->node == c2->node && 
+	        c1->mptr == c2->mptr &&
+	        c1->occ == c2->occ)? 0: 1;
 }
 
 static int init_exec_dds (exec_t* e, qse_mmgr_t* mmgr)
diff --git a/qse/samples/cmn/rex01.c b/qse/samples/cmn/rex01.c
index 9b30f0c4..cd236042 100644
--- a/qse/samples/cmn/rex01.c
+++ b/qse/samples/cmn/rex01.c
@@ -26,6 +26,8 @@ static int rex_main (int argc, qse_char_t* argv[])
 		return -1;
 	}
 
+qse_rex_setoption (rex, QSE_REX_STRICT);
+
 	start = qse_rex_comp (rex, argv[1], qse_strlen(argv[1]));
 	if (start == QSE_NULL)
 	{