diff --git a/app/logical/post_query/parser.rb b/app/logical/post_query/parser.rb
index 73dd91de6..165a9ba37 100644
--- a/app/logical/post_query/parser.rb
+++ b/app/logical/post_query/parser.rb
@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "strscan"
-
 # A PostQuery::Parser parses a search string into a PostQuery::AST.
 #
 # @example
@@ -27,27 +25,19 @@ require "strscan"
 #               | "'" /[^']+/ "'"
 # tag           = /[^ *]+/
 # wildcard      = /[^ ]+/
-#
-# Ref:
-#
-# * https://hmac.dev/posts/2019-05-19-ruby-parser-combinators.html
 
 class PostQuery
   class Parser
     extend Memoist
 
-    class Error < StandardError; end
-
     METATAG_NAME_REGEX = /(#{PostQueryBuilder::METATAGS.join("|")}):/i
 
-    attr_reader :input
-    private attr_reader :scanner, :unclosed_parens
+    attr_reader :parser
+    delegate :error, :rest, :eos?, :accept, :expect, :rewind, :zero_or_more, :one_or_more, :one_of, to: :parser
 
     # @param input [String] The search string to parse.
     def initialize(input)
-      @input = input.to_s.clone.freeze
-      @scanner = StringScanner.new(@input)
-      @unclosed_parens = 0
+      @parser = StringParser.new(input, state: 0) # 0 is the initial number of unclosed parens.
     end
 
     # Parse a search and return the AST.
@@ -64,7 +54,7 @@ class PostQuery
       # @return [PostQuery::AST] The AST of the parsed search.
       def parse
         parse!
-      rescue Error
+      rescue StringParser::Error
         AST.none
       end
 
@@ -73,8 +63,8 @@ class PostQuery
       # @return [PostQuery::AST] The AST of the parsed search.
       def parse!
         ast = root
-        raise Error, "Unexpected EOS (rest: '#{scanner.rest}')" unless scanner.eos?
-        raise Error, "Unclosed parentheses (#{@unclosed_parens})" unless @unclosed_parens == 0
+        error("Unexpected EOS (rest: '#{rest}')") unless eos?
+        error("Unclosed parentheses (#{unclosed_parens})") unless unclosed_parens == 0
         ast
       end
 
@@ -144,10 +134,10 @@ class PostQuery
         space
 
         if accept("(")
-          @unclosed_parens += 1
+          self.unclosed_parens += 1
           a = or_clause
           expect(")")
-          @unclosed_parens -= 1
+          self.unclosed_parens -= 1
           a
         else
           term
@@ -189,7 +179,7 @@ class PostQuery
       # A wildcard is a string that contains a '*' character and that begins with a nonspace, non-')', non-'~', or non-'-' character, followed by nonspace characters.
       def wildcard
         t = string(/(?=[^ ]*\*)[^ \)~-][^ ]*/, skip_balanced_parens: true)
-        raise Error if t.match?(/\A#{METATAG_NAME_REGEX}/)
+        error("Invalid tag name: #{t}") if t.match?(/\A#{METATAG_NAME_REGEX}/)
         space
         AST.wildcard(t)
       end
@@ -197,7 +187,7 @@ class PostQuery
       # A tag is a string that begins with a nonspace, non-')', non-'~', or non-'-' character, followed by nonspace characters.
       def tag
         t = string(/[^ \)~-][^ ]*/, skip_balanced_parens: true)
-        raise Error if t.downcase.in?(%w[and or]) || t.include?("*") || t.match?(/\A#{METATAG_NAME_REGEX}/)
+        error("Invalid tag name: #{t}") if t.downcase.in?(%w[and or]) || t.include?("*") || t.match?(/\A#{METATAG_NAME_REGEX}/)
         space
         AST.tag(t)
       end
@@ -206,11 +196,11 @@ class PostQuery
         str = expect(pattern)
 
         # XXX: Now put back any trailing right parens we mistakenly consumed.
-        n = @unclosed_parens
+        n = unclosed_parens
         while n > 0 && str.ends_with?(")")
           break if skip_balanced_parens && (str.has_balanced_parens? || str.in?(Tag::PERMITTED_UNBALANCED_TAGS))
           str.chop!
-          scanner.pos -= 1
+          rewind
           n -= 1
         end
 
@@ -222,66 +212,14 @@ class PostQuery
       end
     end
 
-    concerning :HelperMethods do
-      private
+    # The current number of '(' characters without a matching ')'. Used for
+    # determining whether a trailing ')' is part of a tag or not.
+    private def unclosed_parens
+      parser.state
+    end
 
-      # Try to match `pattern`, returning the string if it matched or nil if it didn't.
-      #
-      # @param pattern [Regexp, String] The pattern to match.
-      # @return [String, nil] The matched string, or nil
-      def accept(pattern)
-        @scanner.scan(pattern)
-      end
-
-      # Try to match `pattern`, returning the string if it matched or raising an Error if it didn't.
-      #
-      # @param pattern [Regexp, String] The pattern to match.
-      # @return [String] The matched string
-      def expect(pattern)
-        str = accept(pattern)
-        raise Error, "Expected '#{pattern}'; got '#{str}'" if str.nil?
-        str
-      end
-
-      # Try to parse the given block, backtracking to the original state if the parse failed.
-      def backtrack(&block)
-        saved_pos = @scanner.pos
-        saved_unclosed_parens = @unclosed_parens
-        raise Error if @scanner.eos?
-        yield
-      rescue Error
-        @scanner.pos = saved_pos
-        @unclosed_parens = saved_unclosed_parens
-        raise
-      end
-
-      # Parse the block zero or more times, returning an array of parse results.
-      def zero_or_more(&block)
-        matches = []
-        loop do
-          matches << backtrack { yield }
-        end
-      rescue Error
-        matches
-      end
-
-      # Parse the block one or more times, returning an array of parse results.
-      def one_or_more(&block)
-        first = yield
-        rest = zero_or_more(&block)
-        [first, *rest]
-      end
-
-      # Given a list of parsers, return the first one that succeeds.
-      def one_of(parsers)
-        parsers.each do |parser|
-          return backtrack { parser.call }
-        rescue Error
-          next
-        end
-
-        raise Error, "expected one of: #{parsers}"
-      end
+    private def unclosed_parens=(n)
+      parser.state = n
     end
 
     memoize :parse, :parse!
diff --git a/app/logical/string_parser.rb b/app/logical/string_parser.rb
new file mode 100644
index 000000000..5a0d92229
--- /dev/null
+++ b/app/logical/string_parser.rb
@@ -0,0 +1,101 @@
+# frozen_string_literal: true
+
+require "strscan"
+
+# A StringParser is a wrapper around StringScanner that adds extra
+# helper methods for writing parser-combinator style parsers.
+#
+# @see StringScanner
+# @see https://hmac.dev/posts/2019-05-19-ruby-parser-combinators.html
+class StringParser
+  class Error < StandardError; end
+
+  attr_reader :input
+  attr_accessor :state
+  private attr_reader :scanner
+
+  delegate :rest, :eos?, to: :scanner
+
+  # @param input [String] The string to parse.
+  # @param state [Object] An arbitrary piece of user-defined state. Will be
+  #   rolled back when the parser backtracks or is reset.
+  def initialize(input, state: nil)
+    @input = input.to_s.clone.freeze
+    @state = state
+    @scanner = StringScanner.new(@input)
+  end
+
+  # Try to match `pattern`, returning the string if it matched or nil if it didn't.
+  #
+  # @param pattern [Regexp, String] The pattern to match.
+  # @return [String, nil] The matched string, or nil
+  def accept(pattern)
+    scanner.scan(pattern)
+  end
+
+  # Try to match `pattern`, returning the string if it matched or raising an Error if it didn't.
+  #
+  # @param pattern [Regexp, String] The pattern to match.
+  # @return [String] The matched string
+  # @raise [Error] If the pattern didn't match
+  def expect(pattern)
+    str = scanner.scan(pattern)
+    error("Expected '#{pattern}'; got '#{str}'") if str.nil?
+    str
+  end
+
+  # Move the scan pointer back N characters (default: 1)
+  #
+  # @param n [Integer] The number of characters to move back (default: 1).
+  def rewind(n = 1)
+    scanner.pos -= n
+  end
+
+  # Raise a parse error.
+  #
+  # @param message [String] The parse error message.
+  # @raise [Error]
+  def error(message)
+    raise Error, message
+  end
+
+  # Try to parse the given block, backtracking to the previous state if the parse failed.
+  def backtrack(&block)
+    saved_pos = scanner.pos
+    saved_state = state.deep_dup
+    error("Unexpected EOS") if scanner.eos?
+    yield
+  rescue Error
+    scanner.pos = saved_pos
+    self.state = saved_state
+    raise
+  end
+
+  # Parse the block zero or more times, returning an array of parse results.
+  def zero_or_more(&block)
+    matches = []
+    loop do
+      matches << backtrack { yield }
+    end
+  rescue Error
+    matches
+  end
+
+  # Parse the block one or more times, returning an array of parse results.
+  def one_or_more(&block)
+    first = yield
+    rest = zero_or_more(&block)
+    [first, *rest]
+  end
+
+  # Given a list of parsers, try each in sequence and return the first one that succeeds.
+  def one_of(parsers)
+    parsers.each do |parser|
+      return backtrack { parser.call }
+    rescue Error
+      next
+    end
+
+    error("expected one of: #{parsers}")
+  end
+end