log error message when invalid regex is used

see #2178

plugins/inputs/logparser/grok/grok_test.go

@@ -57,6 +57,43 @@ func Benchmark_ParseLine_CustomPattern(b *testing.B) {
 	benchM = m
 }
 
+// Test a very simple parse pattern.
+func TestSimpleParse(t *testing.T) {
+	p := &Parser{
+		Patterns: []string{"%{TESTLOG}"},
+		CustomPatterns: `
+			TESTLOG %{NUMBER:num:int} %{WORD:client}
+		`,
+	}
+	assert.NoError(t, p.Compile())
+
+	m, err := p.ParseLine(`142 bot`)
+	assert.NoError(t, err)
+	require.NotNil(t, m)
+
+	assert.Equal(t,
+		map[string]interface{}{
+			"num":    int64(142),
+			"client": "bot",
+		},
+		m.Fields())
+}
+
+// Verify that patterns with a regex lookahead fail at compile time.
+func TestParsePatternsWithLookahead(t *testing.T) {
+	p := &Parser{
+		Patterns: []string{"%{MYLOG}"},
+		CustomPatterns: `
+			NOBOT ((?!bot|crawl).)*
+			MYLOG %{NUMBER:num:int} %{NOBOT:client}
+		`,
+	}
+	assert.NoError(t, p.Compile())
+
+	_, err := p.ParseLine(`1466004605359052000 bot`)
+	assert.Error(t, err)
+}
+
 func TestMeasurementName(t *testing.T) {
 	p := &Parser{
 		Measurement: "my_web_log",

plugins/inputs/logparser/logparser.go

@@ -226,6 +226,8 @@ func (l *LogParserPlugin) parser() {
 				if m != nil {
 					l.acc.AddFields(m.Name(), m.Fields(), m.Tags(), m.Time())
 				}
+			} else {
+				log.Println("E! Error parsing log line: " + err.Error())
 			}
 		}
 	}