diff --git a/extension.json b/extension.json
index 83e7355a0..6ec2da556 100644
--- a/extension.json
+++ b/extension.json
@@ -159,6 +159,7 @@
 		"AFPException": "includes/parser/AFPException.php",
 		"AFPParserState": "includes/parser/AFPParserState.php",
 		"AFPToken": "includes/parser/AFPToken.php",
+		"AFPTransitionBase": "includes/parser/AFPTransitionBase.php",
 		"AFPTreeNode": "includes/parser/AFPTreeNode.php",
 		"AFPSyntaxTree": "includes/parser/AFPSyntaxTree.php",
 		"AFPTreeParser": "includes/parser/AFPTreeParser.php",
diff --git a/includes/parser/AFPTransitionBase.php b/includes/parser/AFPTransitionBase.php
new file mode 100644
index 000000000..ac90b70b0
--- /dev/null
+++ b/includes/parser/AFPTransitionBase.php
@@ -0,0 +1,151 @@
+<?php
+/**
+ * Base parse-related class to be used while the old parser is being phased out
+ *
+ * @internal This is a temporary class until things are settled down
+ */
+abstract class AFPTransitionBase {
+	public const FUNCTIONS = [
+		'lcase' => 'funcLc',
+		'ucase' => 'funcUc',
+		'length' => 'funcLen',
+		'string' => 'castString',
+		'int' => 'castInt',
+		'float' => 'castFloat',
+		'bool' => 'castBool',
+		'norm' => 'funcNorm',
+		'ccnorm' => 'funcCCNorm',
+		'ccnorm_contains_any' => 'funcCCNormContainsAny',
+		'ccnorm_contains_all' => 'funcCCNormContainsAll',
+		'specialratio' => 'funcSpecialRatio',
+		'rmspecials' => 'funcRMSpecials',
+		'rmdoubles' => 'funcRMDoubles',
+		'rmwhitespace' => 'funcRMWhitespace',
+		'count' => 'funcCount',
+		'rcount' => 'funcRCount',
+		'get_matches' => 'funcGetMatches',
+		'ip_in_range' => 'funcIPInRange',
+		'contains_any' => 'funcContainsAny',
+		'contains_all' => 'funcContainsAll',
+		'equals_to_any' => 'funcEqualsToAny',
+		'substr' => 'funcSubstr',
+		'strlen' => 'funcLen',
+		'strpos' => 'funcStrPos',
+		'str_replace' => 'funcStrReplace',
+		'rescape' => 'funcStrRegexEscape',
+		'set' => 'funcSetVar',
+		'set_var' => 'funcSetVar',
+		'sanitize' => 'funcSanitize',
+	];
+
+	/**
+	 * The minimum and maximum amount of arguments required by each function.
+	 * @var int[][]
+	 */
+	const FUNC_ARG_COUNT = [
+		'lcase' => [ 1, 1 ],
+		'ucase' => [ 1, 1 ],
+		'length' => [ 1, 1 ],
+		'string' => [ 1, 1 ],
+		'int' => [ 1, 1 ],
+		'float' => [ 1, 1 ],
+		'bool' => [ 1, 1 ],
+		'norm' => [ 1, 1 ],
+		'ccnorm' => [ 1, 1 ],
+		'ccnorm_contains_any' => [ 2, INF ],
+		'ccnorm_contains_all' => [ 2, INF ],
+		'specialratio' => [ 1, 1 ],
+		'rmspecials' => [ 1, 1 ],
+		'rmdoubles' => [ 1, 1 ],
+		'rmwhitespace' => [ 1, 1 ],
+		'count' => [ 1, 2 ],
+		'rcount' => [ 1, 2 ],
+		'get_matches' => [ 2, 2 ],
+		'ip_in_range' => [ 2, 2 ],
+		'contains_any' => [ 2, INF ],
+		'contains_all' => [ 2, INF ],
+		'equals_to_any' => [ 2, INF ],
+		'substr' => [ 2, 3 ],
+		'strlen' => [ 1, 1 ],
+		'strpos' => [ 2, 3 ],
+		'str_replace' => [ 3, 3 ],
+		'rescape' => [ 1, 1 ],
+		'set' => [ 2, 2 ],
+		'set_var' => [ 2, 2 ],
+		'sanitize' => [ 1, 1 ],
+	];
+
+	/**
+	 * @var int The position of the current token
+	 */
+	protected $mPos;
+
+	/**
+	 * Check that a built-in function has been provided the right amount of arguments
+	 *
+	 * @param array $args The arguments supplied to the function
+	 * @param string $func The function name
+	 * @throws AFPUserVisibleException
+	 */
+	protected function checkArgCount( $args, $func ) {
+		if ( !array_key_exists( $func, self::FUNC_ARG_COUNT ) ) {
+			throw new InvalidArgumentException( "$func is not a valid function." );
+		}
+		list( $min, $max ) = self::FUNC_ARG_COUNT[ $func ];
+		if ( count( $args ) < $min ) {
+			throw new AFPUserVisibleException(
+				$min === 1 ? 'noparams' : 'notenoughargs',
+				$this->mPos,
+				[ $func, $min, count( $args ) ]
+			);
+		} elseif ( count( $args ) > $max ) {
+			throw new AFPUserVisibleException(
+				'toomanyargs',
+				$this->mPos,
+				[ $func, $max, count( $args ) ]
+			);
+		}
+	}
+
+	/**
+	 * Check whether the given name is a reserved identifier, e.g. the name of a built-in variable,
+	 * function, or keyword.
+	 *
+	 * @param string $name
+	 * @return bool
+	 */
+	protected function isReservedIdentifier( $name ) {
+		return $this->isBuiltinVar( $name ) ||
+			array_key_exists( $name, self::FUNCTIONS ) ||
+			// We need to check for true, false, if/then/else etc. because, even if they have a different
+			// AFPToken type, they may be used inside set/set_var()
+			in_array( $name, AbuseFilterTokenizer::KEYWORDS, true );
+	}
+
+	/**
+	 * Check whether the given name refers to a built-in variable, including
+	 * deprecated and disabled variables.
+	 *
+	 * @param string $varname
+	 * @return bool
+	 */
+	protected function isBuiltinVar( $varname ) {
+		$builderValues = AbuseFilter::getBuilderValues();
+		$deprecatedVars = AbuseFilter::getDeprecatedVariables();
+
+		return array_key_exists( $varname, $builderValues['vars'] ) ||
+			array_key_exists( $varname, AbuseFilter::DISABLED_VARS ) ||
+			array_key_exists( $varname, $deprecatedVars );
+	}
+
+	/**
+	 * @param string $fname
+	 * @return bool
+	 */
+	protected function functionIsVariadic( $fname ) {
+		if ( !array_key_exists( $fname, self::FUNC_ARG_COUNT ) ) {
+			throw new InvalidArgumentException( "Function $fname is not valid" );
+		}
+		return self::FUNC_ARG_COUNT[$fname][1] === INF;
+	}
+}
diff --git a/includes/parser/AFPTreeParser.php b/includes/parser/AFPTreeParser.php
index 340fbf125..5adf3be3d 100644
--- a/includes/parser/AFPTreeParser.php
+++ b/includes/parser/AFPTreeParser.php
@@ -13,7 +13,7 @@ use Psr\Log\LoggerInterface;
 /**
  * A parser that transforms the text of the filter into a parse tree.
  */
-class AFPTreeParser {
+class AFPTreeParser extends AFPTransitionBase {
 	/**
 	 * @var array[] Contains the AFPTokens for the code being parsed
 	 */
@@ -22,10 +22,6 @@ class AFPTreeParser {
 	 * @var AFPToken The current token
 	 */
 	public $mCur;
-	/**
-	 * @var int The position of the current token
-	 */
-	public $mPos;
 	/**
 	 * @var string|null The ID of the filter being parsed, if available. Can also be "global-$ID"
 	 */
@@ -735,46 +731,4 @@ class AFPTreeParser {
 		$this->move();
 		return $result;
 	}
-
-	/**
-	 * Check that a built-in function has been provided the right amount of arguments
-	 *
-	 * @param array $args The arguments supplied to the function
-	 * @param string $func The function name
-	 * @throws AFPUserVisibleException
-	 * @see AbuseFilterParser::checkArgCount()
-	 * @todo This is a duplicate of AbuseFilter::checkEnoughArguments, and such duplication
-	 * should be avoided when merging the parsers.
-	 */
-	protected function checkArgCount( $args, $func ) {
-		if ( !array_key_exists( $func, AbuseFilterParser::FUNC_ARG_COUNT ) ) {
-			throw new InvalidArgumentException( "$func is not a valid function." );
-		}
-		list( $min, $max ) = AbuseFilterParser::FUNC_ARG_COUNT[ $func ];
-		if ( count( $args ) < $min ) {
-			throw new AFPUserVisibleException(
-				$min === 1 ? 'noparams' : 'notenoughargs',
-				$this->mCur->pos,
-				[ $func, $min, count( $args ) ]
-			);
-		} elseif ( count( $args ) > $max ) {
-			throw new AFPUserVisibleException(
-				'toomanyargs',
-				$this->mCur->pos,
-				[ $func, $max, count( $args ) ]
-			);
-		}
-	}
-
-	/**
-	 * @param string $fname
-	 * @return bool
-	 * @see AbuseFilterParser::functionIsVariadic
-	 */
-	protected function functionIsVariadic( $fname ) {
-		if ( !array_key_exists( $fname, AbuseFilterParser::FUNC_ARG_COUNT ) ) {
-			throw new InvalidArgumentException( "Function $fname is not valid" );
-		}
-		return AbuseFilterParser::FUNC_ARG_COUNT[$fname][1] === INF;
-	}
 }
diff --git a/includes/parser/AbuseFilterParser.php b/includes/parser/AbuseFilterParser.php
index 1e8eb26af..42cbc4f58 100644
--- a/includes/parser/AbuseFilterParser.php
+++ b/includes/parser/AbuseFilterParser.php
@@ -3,15 +3,11 @@
 use Psr\Log\LoggerInterface;
 use Wikimedia\Equivset\Equivset;
 
-class AbuseFilterParser {
+class AbuseFilterParser extends AFPTransitionBase {
 	/**
 	 * @var array[] Contains the AFPTokens for the code being parsed
 	 */
 	public $mTokens;
-	/**
-	 * @var int The position of the current token
-	 */
-	public $mPos;
 	/**
 	 * @var bool Are we inside a short circuit evaluation?
 	 */
@@ -65,76 +61,6 @@ class AbuseFilterParser {
 	 */
 	protected $statsd;
 
-	public const FUNCTIONS = [
-		'lcase' => 'funcLc',
-		'ucase' => 'funcUc',
-		'length' => 'funcLen',
-		'string' => 'castString',
-		'int' => 'castInt',
-		'float' => 'castFloat',
-		'bool' => 'castBool',
-		'norm' => 'funcNorm',
-		'ccnorm' => 'funcCCNorm',
-		'ccnorm_contains_any' => 'funcCCNormContainsAny',
-		'ccnorm_contains_all' => 'funcCCNormContainsAll',
-		'specialratio' => 'funcSpecialRatio',
-		'rmspecials' => 'funcRMSpecials',
-		'rmdoubles' => 'funcRMDoubles',
-		'rmwhitespace' => 'funcRMWhitespace',
-		'count' => 'funcCount',
-		'rcount' => 'funcRCount',
-		'get_matches' => 'funcGetMatches',
-		'ip_in_range' => 'funcIPInRange',
-		'contains_any' => 'funcContainsAny',
-		'contains_all' => 'funcContainsAll',
-		'equals_to_any' => 'funcEqualsToAny',
-		'substr' => 'funcSubstr',
-		'strlen' => 'funcLen',
-		'strpos' => 'funcStrPos',
-		'str_replace' => 'funcStrReplace',
-		'rescape' => 'funcStrRegexEscape',
-		'set' => 'funcSetVar',
-		'set_var' => 'funcSetVar',
-		'sanitize' => 'funcSanitize',
-	];
-
-	/**
-	 * The minimum and maximum amount of arguments required by each function.
-	 * @var int[][]
-	 */
-	const FUNC_ARG_COUNT = [
-		'lcase' => [ 1, 1 ],
-		'ucase' => [ 1, 1 ],
-		'length' => [ 1, 1 ],
-		'string' => [ 1, 1 ],
-		'int' => [ 1, 1 ],
-		'float' => [ 1, 1 ],
-		'bool' => [ 1, 1 ],
-		'norm' => [ 1, 1 ],
-		'ccnorm' => [ 1, 1 ],
-		'ccnorm_contains_any' => [ 2, INF ],
-		'ccnorm_contains_all' => [ 2, INF ],
-		'specialratio' => [ 1, 1 ],
-		'rmspecials' => [ 1, 1 ],
-		'rmdoubles' => [ 1, 1 ],
-		'rmwhitespace' => [ 1, 1 ],
-		'count' => [ 1, 2 ],
-		'rcount' => [ 1, 2 ],
-		'get_matches' => [ 2, 2 ],
-		'ip_in_range' => [ 2, 2 ],
-		'contains_any' => [ 2, INF ],
-		'contains_all' => [ 2, INF ],
-		'equals_to_any' => [ 2, INF ],
-		'substr' => [ 2, 3 ],
-		'strlen' => [ 1, 1 ],
-		'strpos' => [ 2, 3 ],
-		'str_replace' => [ 3, 3 ],
-		'rescape' => [ 1, 1 ],
-		'set' => [ 2, 2 ],
-		'set_var' => [ 2, 2 ],
-		'sanitize' => [ 1, 1 ],
-	];
-
 	// Functions that affect parser state, and shouldn't be cached.
 	public const ACTIVE_FUNCTIONS = [
 		'funcSetVar',
@@ -1166,37 +1092,6 @@ class AbuseFilterParser {
 		return $this->mVariables->getVar( $var, $flags, $this->mFilter );
 	}
 
-	/**
-	 * Check whether the given name refers to a built-in variable, including
-	 * deprecated and disabled variables.
-	 *
-	 * @param string $varname
-	 * @return bool
-	 */
-	protected function isBuiltinVar( $varname ) {
-		$builderValues = AbuseFilter::getBuilderValues();
-		$deprecatedVars = AbuseFilter::getDeprecatedVariables();
-
-		return array_key_exists( $varname, $builderValues['vars'] ) ||
-			array_key_exists( $varname, AbuseFilter::DISABLED_VARS ) ||
-			array_key_exists( $varname, $deprecatedVars );
-	}
-
-	/**
-	 * Check whether the given name is a reserved identifier, e.g. the name of a built-in variable,
-	 * function, or keyword.
-	 *
-	 * @param string $name
-	 * @return bool
-	 */
-	protected function isReservedIdentifier( $name ) {
-		return $this->isBuiltinVar( $name ) ||
-			array_key_exists( $name, self::FUNCTIONS ) ||
-			// We need to check for true, false, if/then/else etc. because, even if they have a different
-			// AFPToken type, they may be used inside set/set_var()
-			in_array( $name, AbuseFilterTokenizer::KEYWORDS, true );
-	}
-
 	/**
 	 * @param string $name
 	 * @param mixed $value
@@ -1209,33 +1104,6 @@ class AbuseFilterParser {
 		$this->mVariables->setVar( $name, $value );
 	}
 
-	/**
-	 * Check that a built-in function has been provided the right amount of arguments
-	 *
-	 * @param array $args The arguments supplied to the function
-	 * @param string $func The function name
-	 * @throws AFPUserVisibleException
-	 */
-	protected function checkArgCount( $args, $func ) {
-		if ( !array_key_exists( $func, self::FUNC_ARG_COUNT ) ) {
-			throw new InvalidArgumentException( "$func is not a valid function." );
-		}
-		list( $min, $max ) = self::FUNC_ARG_COUNT[ $func ];
-		if ( count( $args ) < $min ) {
-			throw new AFPUserVisibleException(
-				$min === 1 ? 'noparams' : 'notenoughargs',
-				$this->mCur->pos,
-				[ $func, $min, count( $args ) ]
-			);
-		} elseif ( count( $args ) > $max ) {
-			throw new AFPUserVisibleException(
-				'toomanyargs',
-				$this->mCur->pos,
-				[ $func, $max, count( $args ) ]
-			);
-		}
-	}
-
 	/**
 	 * Helper to call a built-in function.
 	 *
@@ -1964,15 +1832,4 @@ class AbuseFilterParser {
 		}
 	}
 
-	/**
-	 * @param string $fname
-	 * @return bool
-	 * @see AFPTreeParser::functionIsVariadic
-	 */
-	protected function functionIsVariadic( $fname ) {
-		if ( !array_key_exists( $fname, self::FUNC_ARG_COUNT ) ) {
-			throw new InvalidArgumentException( "Function $fname is not valid" );
-		}
-		return self::FUNC_ARG_COUNT[$fname][1] === INF;
-	}
 }
diff --git a/tests/phpunit/AbuseFilterParserEquivsetTest.php b/tests/phpunit/AbuseFilterParserEquivsetTest.php
index cab20bc4c..b86aeaabc 100644
--- a/tests/phpunit/AbuseFilterParserEquivsetTest.php
+++ b/tests/phpunit/AbuseFilterParserEquivsetTest.php
@@ -25,6 +25,7 @@
  *
  * @covers AbuseFilterCachingParser
  * @covers AFPTreeParser
+ * @covers AFPTransitionBase
  * @covers AFPTreeNode
  * @covers AFPParserState
  * @covers AbuseFilterParser
diff --git a/tests/phpunit/unit/AbuseFilterParserTest.php b/tests/phpunit/unit/AbuseFilterParserTest.php
index c4dd1343d..d44e2cff6 100644
--- a/tests/phpunit/unit/AbuseFilterParserTest.php
+++ b/tests/phpunit/unit/AbuseFilterParserTest.php
@@ -30,6 +30,7 @@ use Psr\Log\NullLogger;
  *
  * @covers AbuseFilterCachingParser
  * @covers AFPTreeParser
+ * @covers AFPTransitionBase
  * @covers AFPTreeNode
  * @covers AFPParserState
  * @covers AbuseFilterParser