diff --git a/lib/fluent/plugin/in_syslog.rb b/lib/fluent/plugin/in_syslog.rb
index 704b801932..665aeff0f5 100644
--- a/lib/fluent/plugin/in_syslog.rb
+++ b/lib/fluent/plugin/in_syslog.rb
@@ -27,7 +27,7 @@ class SyslogInput < Input
     helpers :parser, :compat_parameters, :server
 
     DEFAULT_PARSER = 'syslog'
-    SYSLOG_REGEXP = /^\<([0-9]+)\>(.*)/
+    SYSLOG_REGEXP = /\A\<([0-9]+)\>(.*)/
 
     FACILITY_MAP = {
       0   => 'kern',
diff --git a/lib/fluent/plugin/parser_syslog.rb b/lib/fluent/plugin/parser_syslog.rb
index 0fefb2e2f1..52c3b1f066 100644
--- a/lib/fluent/plugin/parser_syslog.rb
+++ b/lib/fluent/plugin/parser_syslog.rb
@@ -38,15 +38,15 @@ class SyslogParser < Parser
         \A<(?<pri>[0-9]{1,3})\>[1-9]\d{0,2} %s\z
       EOS
 
-      REGEXP_DETECT_RFC5424 = /^\<[0-9]{1,3}\>[1-9]\d{0,2}/
+      REGEXP_DETECT_RFC5424 = /\A\<[0-9]{1,3}\>[1-9]\d{0,2}/
 
       RFC3164_WITHOUT_TIME_AND_PRI_REGEXP = /(?<host>[^ ]*) (?<ident>[^ :\[]*)(?:\[(?<pid>[0-9]+)\])?(?:[^\:]*\:)? *(?<message>.*)$/
       RFC3164_CAPTURES = RFC3164_WITHOUT_TIME_AND_PRI_REGEXP.names.freeze
-      RFC3164_PRI_REGEXP = /^<(?<pri>[0-9]{1,3})>/
+      RFC3164_PRI_REGEXP = /\A<(?<pri>[0-9]{1,3})>/
 
       RFC5424_WITHOUT_TIME_AND_PRI_REGEXP = /(?<host>[!-~]{1,255}) (?<ident>[!-~]{1,48}) (?<pid>[!-~]{1,128}) (?<msgid>[!-~]{1,32}) (?<extradata>(?:\-|(?:\[.*?(?<!\\)\])))(?: (?<message>.+))?\z/m
       RFC5424_CAPTURES = RFC5424_WITHOUT_TIME_AND_PRI_REGEXP.names.freeze
-      RFC5424_PRI_REGEXP = /^<(?<pri>\d{1,3})>\d\d{0,2}\s/
+      RFC5424_PRI_REGEXP = /\A<(?<pri>\d{1,3})>\d\d{0,2}\s/
 
       config_set_default :time_format, "%b %d %H:%M:%S"
       desc 'If the incoming logs have priority prefix, e.g. <9>, set true'
diff --git a/test/plugin/test_in_syslog.rb b/test/plugin/test_in_syslog.rb
index bc52eecdc2..df8e0c9661 100755
--- a/test/plugin/test_in_syslog.rb
+++ b/test/plugin/test_in_syslog.rb
@@ -197,6 +197,27 @@ def test_space_after_input_owned_priority_preserves_input_priority_syntax(parser
     assert_equal(messages[2], d.events[2][2]['unmatched_line'])
   end
 
+  def test_priority_on_a_later_line_is_unmatched_when_input_parses_priority
+    d = create_driver([
+      ipv4_config,
+      'emit_unmatched_lines true',
+      '<parse>',
+      '  @type none',
+      '</parse>',
+    ].join("\n"))
+
+    message = "first line\n<14>second line"
+    d.run(expect_emits: 1) do
+      u = UDPSocket.new
+      u.connect('127.0.0.1', @port)
+      u.send(message, 0)
+    end
+
+    assert_equal(1, d.events.size)
+    assert_equal('syslog.unmatched', d.events[0][0])
+    assert_equal(message, d.events[0][2]['unmatched_line'])
+  end
+
   def test_msg_size
     d = create_driver
     tests = create_test_case
diff --git a/test/plugin/test_parser_syslog.rb b/test/plugin/test_parser_syslog.rb
index 9d55c4f7d4..a96dd360ed 100644
--- a/test/plugin/test_parser_syslog.rb
+++ b/test/plugin/test_parser_syslog.rb
@@ -318,6 +318,42 @@ def test_truncated_priority_or_timestamp_is_rejected_without_raising(data)
     assert_input_rejected(text, :rejects)
   end
 
+  data(
+    'regexp/rfc5424' => ['regexp', 'rfc5424', "first line\n<14>1 2026-04-25T16:43:29Z host app 123 ID - second line"],
+    'string/rfc5424' => ['string', 'rfc5424', "first line\n<14>1 2026-04-25T16:43:29Z host app 123 ID - second line"],
+    'regexp/rfc3164' => ['regexp', 'rfc3164', "first line\n<14>Apr 25 16:43:29 host app: second line"],
+    'string/rfc3164' => ['string', 'rfc3164', "first line\n<14>Apr 25 16:43:29 host app: second line"],
+    'regexp/auto' => ['regexp', 'auto', "first line\n<14>1 2026-04-25T16:43:29Z host app 123 ID - second line"],
+    'string/auto' => ['string', 'auto', "first line\n<14>1 2026-04-25T16:43:29Z host app 123 ID - second line"],
+  )
+  def test_priority_on_a_later_line_is_rejected(data)
+    parser_engine, message_format, text = data
+    @parser.configure(
+      'parser_engine' => parser_engine,
+      'message_format' => message_format,
+      'with_priority' => true,
+    )
+
+    assert_input_rejected(text, :rejects)
+  end
+
+  data('regexp' => 'regexp', 'string' => 'string')
+  def test_auto_detects_rfc3164_when_a_later_line_looks_like_rfc5424(parser_engine)
+    @parser.configure(
+      'parser_engine' => parser_engine,
+      'message_format' => 'auto',
+      'with_priority' => true,
+    )
+
+    text = "<13>Apr 25 16:43:29 web01 nginx: request failed:\n<14>1 2026-04-25T16:43:29Z host app 123 ID - second line"
+    time = record = nil
+    @parser.instance.parse(text) do |t, r|
+      time, record = t, r
+    end
+    assert_equal(event_time('Apr 25 16:43:29', format: '%b %d %H:%M:%S'), time)
+    assert_equal([13, 'web01', 'nginx'], record.values_at('pri', 'host', 'ident'))
+  end
+
   data(
     'rfc5424' => ['rfc5424', 'time_format'],
     'auto' => ['auto', 'rfc5424_time_format'],