diff --git a/regress/Makefile b/regress/Makefile
index 9f97c2bde9f0..4e2e1c708b55 100644
--- a/regress/Makefile
+++ b/regress/Makefile
@@ -1,4 +1,4 @@
-# $NetBSD: Makefile,v 1.21 2018/11/10 10:40:55 rillig Exp $
+# $NetBSD: Makefile,v 1.22 2019/03/21 21:45:30 rillig Exp $
 #
 # See https://www.netbsd.org/docs/pkgsrc/regression.html for more
 # information about these tests.
@@ -15,6 +15,7 @@ SUBDIR+=	check-portability
 SUBDIR+=	compiler
 SUBDIR+=	env-vars
 SUBDIR+=	ignore-tools
+SUBDIR+=	infra-unittests
 SUBDIR+=	make-env-phases
 SUBDIR+=	make-quoting
 SUBDIR+=	pkg-options
diff --git a/regress/infra-unittests/mocked-include.sh b/regress/infra-unittests/mocked-include.sh
new file mode 100644
index 000000000000..cec97a319b1e
--- /dev/null
+++ b/regress/infra-unittests/mocked-include.sh
@@ -0,0 +1,20 @@
+#! /bin/sh
+set -eu
+
+# Ensures that files that are overridden in the "mocked pkgsrc" directory
+# are picked up before those from the "real pkgsrc" directory.
+
+. "./test.subr"
+
+create_file "including.mk" <<EOF
+.include "mk/bsd.prefs.mk"
+EOF
+
+create_pkgsrc_file "mk/bsd.prefs.mk" <<EOF
+all:
+	@echo 'the mocked definition wins'
+EOF
+
+out=$(test_file "including.mk")
+
+assert_that "$out" --equals "the mocked definition wins"
diff --git a/regress/infra-unittests/spec b/regress/infra-unittests/spec
new file mode 100644
index 000000000000..b7bde4b37173
--- /dev/null
+++ b/regress/infra-unittests/spec
@@ -0,0 +1,12 @@
+# $NetBSD: spec,v 1.1 2019/03/21 21:45:30 rillig Exp $
+#
+# Unit tests for the pkgsrc infrastructure.
+
+do_test() {
+	sh ./mocked-include.sh || TEST_EXITSTATUS=$?
+	sh ./tools-bison.sh || TEST_EXITSTATUS=$?
+}
+
+check_result() {
+	exit_status 0
+}
diff --git a/regress/infra-unittests/test.subr b/regress/infra-unittests/test.subr
new file mode 100644
index 000000000000..edf3fc43019d
--- /dev/null
+++ b/regress/infra-unittests/test.subr
@@ -0,0 +1,146 @@
+#! /bin/sh
+# $NetBSD: test.subr,v 1.1 2019/03/21 21:45:30 rillig Exp $
+set -eu
+
+# This file defines utilities for testing Makefile fragments in a mocked
+# environment. It is used primarily to test the pkgsrc infrastructure.
+#
+# It defines the following shell variables:
+#
+#	cleanup
+#		If yes (the default), clean up the temporary directory
+#		after the test has run successfully.
+#
+# It defines the following shell functions:
+#
+#	mock_cmd
+#		Returns the path to a newly created shell program whose
+#		behavior (output and exit status) is specified by pairs
+#		of --when-args/--then-output or --when-args/--then-exit.
+#		Example:
+#
+#		hello=$(mock_cmd mock-hello \
+#			--when-args "" --then-output "Hello, world!" \
+#			--when-args "-t" --then-output "hello, world" \
+#			--when-args "-?" --then-exit 1
+#		)
+#
+#	create_file $filename <<EOF ... EOF
+#		Creates a file in the temporary directory. The filename
+#		is relative to the temporary directory.
+#
+#	create_pkgsrc_file $filename <<EOF ... EOF
+#		Creates a file in the temporary pkgsrc directory. This
+#		file will be included instead of the one in the real
+#		pkgsrc infrastructure. The filename is relative to the
+#		pkgsrc directory, such as mk/defaults/mk.conf.
+#
+#		This is typically used for creating an empty file for
+#		mk/bsd.prefs.mk or similar files that are included by the
+#		file that is currently under test.
+#
+#	test_file $filename $bmake_args...
+#		Runs bmake with the correct environment so that it picks
+#		up the mocked infrastructure files first and then the ones
+#		from the real pkgsrc installation. The filename is
+#		relative to the temporary directory.
+#
+#	assert_that $actual --equals $expected
+#		Complains loudly if $actual is not equal to $expected.
+
+: ${cleanup:=yes}
+tmpdir="${TMP:-/tmp}/infra-unittests-$$"
+mocked_pkgsrcdir="$tmpdir/pkgsrc"
+rm -rf "$tmpdir"
+mkdir -p "$mocked_pkgsrcdir"
+
+real_pkgsrcdir=""
+for rel in .. ../.. ../../..; do
+	[ -f "$rel/mk/bsd.pkg.mk" ] || continue
+	real_pkgsrcdir="$rel"
+done
+[ "$real_pkgsrcdir" ] || {
+	printf 'error: must be run from somewhere inside the pkgsrc directory\n' 1>&2
+	exit 1
+}
+
+maybe_cleanup() {
+	exit_status=$?
+	if [ $exit_status -ne 0 ]; then
+		printf 'info: the test files are in %s\n' "$tmpdir" 1>&2
+		exit $exit_status
+	fi
+
+	[ "$cleanup" = "yes" ] && rm -rf "$tmpdir"
+}
+trap "maybe_cleanup" EXIT
+
+mock_cmd() {
+	cmdname="$1"
+	shift 1
+
+	{
+	printf '#! /bin/sh\n\n'
+	while [ $# -ge 4 ]; do
+		case $1,$3 in
+		(--when-args,--then-output)
+			cat <<EOF
+[ "x\$*" = "x$2" ] && { printf '%s\n' "$4" && exit 0; }
+
+EOF
+			shift 4
+			;;
+		(--when-args,--then-exit)
+			cat <<EOF
+[ "x\$*" = "x$2" ] && exit $4
+
+EOF
+			shift 4
+			;;
+		*)	printf 'error: invalid arguments to mock_cmd: %s\n' "$*" 1>&2
+			exit 1
+			;;
+		esac
+	done
+	cat <<EOF
+printf 'error: %s: no mock behavior defined for arguments %s\n' "\$0" "\$*" 1>&2
+exit 1
+EOF
+	} > "$tmpdir/$cmdname"
+	chmod +x "$tmpdir/$cmdname"
+
+	printf '%s\n' "$tmpdir/$cmdname"
+}
+
+create_file() {
+	cat > "$tmpdir/$1"
+}
+
+create_pkgsrc_file() {
+	mkdir -p "$mocked_pkgsrcdir/$(dirname "$1")"
+	cat > "$mocked_pkgsrcdir/$1"
+}
+
+test_file() {
+	cat <<EOF > "$tmpdir/test.subr.main.mk"
+PKGSRCDIR=	$real_pkgsrcdir
+.PATH:		$mocked_pkgsrcdir
+.PATH:		$real_pkgsrcdir
+.include "$1"
+EOF
+	shift
+
+	bmake -f "$tmpdir/test.subr.main.mk" "$@"
+}
+
+assert_that() {
+	case "$2" in
+	(--equals)
+		[ "x$1" = "x$3" ] && return 0
+		printf 'assertion failed:\nexpected: <%s>\nbut was:  <%s>\n' "$3" "$1" 1>&2
+		exit 1
+	(*)
+		printf 'usage: assert_that <expr> --equals <expr>\n' 1>&2
+		exit 1
+	esac
+}
diff --git a/regress/infra-unittests/tools-bison.sh b/regress/infra-unittests/tools-bison.sh
new file mode 100644
index 000000000000..160328d64a06
--- /dev/null
+++ b/regress/infra-unittests/tools-bison.sh
@@ -0,0 +1,59 @@
+#! /bin/sh
+set -eu
+
+# Tests for mk/tools/bison.mk
+
+. "./test.subr"
+
+bison=$(mock_cmd mock-bison \
+	--when-args "--version" --then-output "bison 1.5"
+)
+
+pkg_admin=$(mock_cmd mock-pkg_admin \
+	--when-args "pmatch bison>=1.0 bison-1.5" --then-exit 0 \
+	--when-args "pmatch bison>=1.1 bison-1.5" --then-exit 0 \
+	--when-args "pmatch bison>=2.0 bison-1.5" --then-exit 1
+)
+
+# A package may add more than one entry to the BISON_REQD list. The
+# platform-provided bison may only be used if all of the BISON_REQD
+# entries are below the platform-provided version.
+#
+create_file "multiple-reqd-entries.mk" <<EOF
+BISON_REQD=		1.0 2.0
+USE_TOOLS=		bison
+TOOLS_PLATFORM.bison=	$tmpdir/mock-bison
+ECHO=			echo
+SED=			sed
+PKG_ADMIN=		$tmpdir/mock-pkg_admin
+
+all:
+	@echo \${_TOOLS_USE_PKGSRC.bison}
+
+.include "mk/tools/bison.mk"
+EOF
+
+out=$(test_file "multiple-reqd-entries.mk")
+
+assert_that "$out" --equals "yes"
+
+# Both required versions are lower than the version of the mocked bison,
+# which is 1.5. Therefore the platform-provided bison can be used.
+#
+create_file "multiple-reqd-entries.mk" <<EOF
+BISON_REQD=		1.0 1.1
+USE_TOOLS=		bison
+TOOLS_PLATFORM.bison=	$tmpdir/mock-bison
+ECHO=			echo
+SED=			sed
+PKG_ADMIN=		$tmpdir/mock-pkg_admin
+
+all:
+	@echo \${_TOOLS_USE_PKGSRC.bison}
+
+.include "mk/tools/bison.mk"
+EOF
+
+out=$(test_file "multiple-reqd-entries.mk")
+
+assert_that "$out" --equals "no"