diff --git a/ext/regexp/regexp.go b/ext/regexp/regexp.go
index 6210147f..1d0aa306 100644
--- a/ext/regexp/regexp.go
+++ b/ext/regexp/regexp.go
@@ -32,11 +32,14 @@ func Register(db *sqlite3.Conn) error {
 		db.CreateFunction("regexp_instr", 3, flags, regexInstr),
 		db.CreateFunction("regexp_instr", 4, flags, regexInstr),
 		db.CreateFunction("regexp_instr", 5, flags, regexInstr),
+		db.CreateFunction("regexp_instr", 6, flags, regexInstr),
 		db.CreateFunction("regexp_substr", 2, flags, regexSubstr),
 		db.CreateFunction("regexp_substr", 3, flags, regexSubstr),
 		db.CreateFunction("regexp_substr", 4, flags, regexSubstr),
+		db.CreateFunction("regexp_substr", 5, flags, regexSubstr),
 		db.CreateFunction("regexp_replace", 3, flags, regexReplace),
-		db.CreateFunction("regexp_replace", 4, flags, regexReplace))
+		db.CreateFunction("regexp_replace", 4, flags, regexReplace),
+		db.CreateFunction("regexp_replace", 5, flags, regexReplace))
 }
 
 func load(ctx sqlite3.Context, i int, expr string) (*regexp.Regexp, error) {
@@ -68,6 +71,7 @@ func regexLike(ctx sqlite3.Context, arg ...sqlite3.Value) {
 		ctx.ResultError(err)
 		return // notest
 	}
+
 	text := arg[0].RawText()
 	ctx.ResultBool(re.Match(text))
 }
@@ -78,10 +82,11 @@ func regexCount(ctx sqlite3.Context, arg ...sqlite3.Value) {
 		ctx.ResultError(err)
 		return // notest
 	}
+
 	text := arg[0].RawText()
 	if len(arg) > 2 {
 		pos := arg[2].Int()
-		_, text = split(text, pos)
+		text = text[skip(text, pos):]
 	}
 	ctx.ResultInt(len(re.FindAll(text, -1)))
 }
@@ -92,26 +97,23 @@ func regexSubstr(ctx sqlite3.Context, arg ...sqlite3.Value) {
 		ctx.ResultError(err)
 		return // notest
 	}
+
 	text := arg[0].RawText()
+	var pos, n, subexpr int
 	if len(arg) > 2 {
-		pos := arg[2].Int()
-		_, text = split(text, pos)
+		pos = arg[2].Int()
 	}
-	n := 0
 	if len(arg) > 3 {
 		n = arg[3].Int()
 	}
+	if len(arg) > 4 {
+		subexpr = arg[4].Int()
+	}
 
-	var res []byte
-	if n <= 1 {
-		res = re.Find(text)
-	} else {
-		all := re.FindAll(text, n)
-		if n <= len(all) {
-			res = all[n-1]
-		}
+	loc := regexFind(re, text, pos, n, subexpr)
+	if loc != nil {
+		ctx.ResultRawText(text[loc[0]:loc[1]])
 	}
-	ctx.ResultRawText(res)
 }
 
 func regexInstr(ctx sqlite3.Context, arg ...sqlite3.Value) {
@@ -120,35 +122,26 @@ func regexInstr(ctx sqlite3.Context, arg ...sqlite3.Value) {
 		ctx.ResultError(err)
 		return // notest
 	}
-	pos := 1
+
 	text := arg[0].RawText()
+	var pos, n, end, subexpr int
 	if len(arg) > 2 {
 		pos = arg[2].Int()
-		_, text = split(text, pos)
 	}
-	n := 0
 	if len(arg) > 3 {
 		n = arg[3].Int()
 	}
-
-	var loc []int
-	if n <= 1 {
-		loc = re.FindIndex(text)
-	} else {
-		all := re.FindAllIndex(text, n)
-		if n <= len(all) {
-			loc = all[n-1]
-		}
+	if len(arg) > 4 && arg[4].Bool() {
+		end = 1
 	}
-	if loc == nil {
-		return
+	if len(arg) > 5 {
+		subexpr = arg[5].Int()
 	}
 
-	end := 0
-	if len(arg) > 4 && arg[4].Bool() {
-		end = 1
+	loc := regexFind(re, text, pos, n, subexpr)
+	if loc != nil {
+		ctx.ResultInt(loc[end] + 1)
 	}
-	ctx.ResultInt(pos + loc[end])
 }
 
 func regexReplace(ctx sqlite3.Context, arg ...sqlite3.Value) {
@@ -157,24 +150,71 @@ func regexReplace(ctx sqlite3.Context, arg ...sqlite3.Value) {
 		ctx.ResultError(err)
 		return // notest
 	}
-	var head, tail []byte
-	tail = arg[0].RawText()
+
+	text := arg[0].RawText()
+	repl := arg[2].RawText()
+	var pos, n int
 	if len(arg) > 3 {
-		pos := arg[3].Int()
-		head, tail = split(tail, pos)
+		pos = arg[3].Int()
+	}
+	if len(arg) > 4 {
+		n = arg[4].Int()
+	}
+
+	res := text
+	pos = skip(text, pos)
+	if n > 0 {
+		all := re.FindAllSubmatchIndex(text[pos:], n)
+		if n <= len(all) {
+			loc := all[n-1]
+			res = text[:pos+loc[0]]
+			res = re.Expand(res, repl, text[pos:], loc)
+			res = append(res, text[pos+loc[1]:]...)
+		}
+	} else {
+		res = append(text[:pos], re.ReplaceAll(text[pos:], repl)...)
 	}
-	tail = re.ReplaceAll(tail, arg[2].RawText())
-	if head != nil {
-		tail = append(head, tail...)
+	ctx.ResultRawText(res)
+}
+
+func regexFind(re *regexp.Regexp, text []byte, pos, n, subexpr int) (loc []int) {
+	pos = skip(text, pos)
+	text = text[pos:]
+
+	if n <= 1 {
+		if subexpr == 0 {
+			loc = re.FindIndex(text)
+		} else {
+			loc = re.FindSubmatchIndex(text)
+		}
+	} else {
+		if subexpr == 0 {
+			all := re.FindAllIndex(text, n)
+			if n <= len(all) {
+				loc = all[n-1]
+			}
+		} else {
+			all := re.FindAllSubmatchIndex(text, n)
+			if n <= len(all) {
+				loc = all[n-1]
+			}
+		}
+	}
+
+	if 2+2*subexpr <= len(loc) {
+		loc = loc[2*subexpr : 2+2*subexpr]
+		loc[0] += pos
+		loc[1] += pos
+		return loc
 	}
-	ctx.ResultRawText(tail)
+	return nil
 }
 
-func split(s []byte, i int) (head, tail []byte) {
-	for pos := range string(s) {
-		if i--; i <= 0 {
-			return s[:pos:pos], s[pos:]
+func skip(text []byte, start int) int {
+	for pos := range string(text) {
+		if start--; start <= 0 {
+			return pos
 		}
 	}
-	return s, nil
+	return len(text)
 }
diff --git a/ext/regexp/regexp_test.go b/ext/regexp/regexp_test.go
index e9d86a79..d8dc83ed 100644
--- a/ext/regexp/regexp_test.go
+++ b/ext/regexp/regexp_test.go
@@ -34,13 +34,33 @@ func TestRegister(t *testing.T) {
 		{`regexp_instr('Hello', 'el.')`, "2"},
 		{`regexp_instr('Hello', '.', 6)`, ""},
 		{`regexp_substr('Hello', 'el.')`, "ell"},
-		{`regexp_substr('Hello', 'l', 2, 2)`, "l"},
 		{`regexp_replace('Hello', 'llo', 'll')`, "Hell"},
-
+		// https://www.postgresql.org/docs/current/functions-matching.html
+		{`regexp_count('ABCABCAXYaxy', 'A.')`, "3"},
+		{`regexp_count('ABCABCAXYaxy', '(?i)A.', 1)`, "4"},
+		{`regexp_instr('number of your street, town zip, FR', '[^,]+', 1, 2)`, "23"},
+		{`regexp_instr('ABCDEFGHI', '(?i)(c..)(...)', 1, 1, 0, 2)`, "6"},
+		{`regexp_substr('number of your street, town zip, FR', '[^,]+', 1, 2)`, " town zip"},
+		{`regexp_substr('ABCDEFGHI', '(?i)(c..)(...)', 1, 1, 2)`, "FGH"},
+		{`regexp_replace('foobarbaz', 'b..', 'X', 1, 1)`, "fooXbaz"},
+		{`regexp_replace('foobarbaz', 'b..', 'X')`, "fooXX"},
+		{`regexp_replace('foobarbaz', 'b(..)', 'X${1}Y')`, "fooXarYXazY"},
+		{`regexp_replace('A PostgreSQL function', '(?i)a|e|i|o|u', 'X', 1, 0)`, "X PXstgrXSQL fXnctXXn"},
+		{`regexp_replace('A PostgreSQL function', '(?i)a|e|i|o|u', 'X', 1, 3)`, "A PostgrXSQL function"},
+		// https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/REGEXP_COUNT.html
 		{`regexp_count('123123123123123', '(12)3', 1)`, "5"},
+		{`regexp_count('123123123123', '123', 3)`, "3"},
+		{`regexp_instr('500 Oracle Parkway, Redwood Shores, CA', '[^ ]+', 1, 6)`, "37"},
 		{`regexp_instr('500 Oracle Parkway, Redwood Shores, CA', '(?i)[s|r|p][[:alpha:]]{6}', 3, 2, 1)`, "28"},
-		{`regexp_substr('500 Oracle Parkway, Redwood Shores, CA', ',[^,]+,', 3, 1)`, ", Redwood Shores,"},
-		{`regexp_replace('500   Oracle     Parkway,    Redwood  Shores, CA', '( ){2,}', ' ', 3)`, "500 Oracle Parkway, Redwood Shores, CA"},
+		{`regexp_instr('1234567890', '(123)(4(56)(78))', 1, 1, 0, 1)`, "1"},
+		{`regexp_instr('1234567890', '(123)(4(56)(78))', 1, 1, 0, 2)`, "4"},
+		{`regexp_instr('1234567890', '(123)(4(56)(78))', 1, 1, 0, 4)`, "7"},
+		{`regexp_substr('500 Oracle Parkway, Redwood Shores, CA', ',[^,]+,')`, ", Redwood Shores,"},
+		{`regexp_substr('http://www.example.com/products', 'http://([[:alnum:]]+\.?){3,4}/?')`, "http://www.example.com/"},
+		{`regexp_substr('1234567890', '(123)(4(56)(78))', 1, 1, 1)`, "123"},
+		{`regexp_substr('1234567890', '(123)(4(56)(78))', 1, 1, 4)`, "78"},
+		{`regexp_substr('123123123123', '1(.)3', 3, 2, 1)`, "2"},
+		{`regexp_replace('500   Oracle     Parkway,    Redwood  Shores, CA', '( ){2,}', ' ')`, "500 Oracle Parkway, Redwood Shores, CA"},
 	}
 
 	for _, tt := range tests {