1 files changed, 93 insertions, 5 deletions
diff --git a/contrib/bmake/unit-tests/varmod-no-match.mk b/contrib/bmake/unit-tests/varmod-no-match.mk
index 2acb27e2e727..c03b4bf94e70 100644
--- a/contrib/bmake/unit-tests/varmod-no-match.mk
+++ b/contrib/bmake/unit-tests/varmod-no-match.mk
@@ -1,9 +1,97 @@
-# $NetBSD: varmod-no-match.mk,v 1.2 2020/08/16 14:25:16 rillig Exp $
+# $NetBSD: varmod-no-match.mk,v 1.3 2023/02/26 06:08:06 rillig Exp $
 #
-# Tests for the :N variable modifier, which filters words that do not match
-# the given pattern.
+# Tests for the expression modifier ':N', which filters words that do not
+# match the given pattern.
+
+
+# Keep all words except for 'two'.
+.if ${:U one two three :Ntwo} != "one three"
+.  error
+.endif
+
+# Keep all words except those starting with 't'.
+# See varmod-match.mk for the details of pattern matching.
+.if ${:U one two three four six :Nt*} != "one four six"
+.  error
+.endif
+
+
+# Idiom: normalize whitespace
+#
+# The modifier ':N' can be used with an empty pattern.  As that pattern never
+# matches a word, the only effect is that the string is split into words and
+# then joined again, thereby normalizing whitespace around and between the
+# words.  And even though the 'N' in ':N' might serve as a mnemonic for
+# "normalize whitespace", this idiom is not used in practice, resorting to the
+# much more common ':M*' to "select all words" instead.
+.if ${:U :N} != ""
+.  error
+.endif
+.if ${:U one    two three :N} != "one two three"
+.  error
+.endif
+.if ${:U one    two three :M*} != "one two three"
+.  error
+.endif
+
+
+# Idiom: single-word expression equals any of several words or patterns
+#
+# If an expression is guaranteed to consist of a single word, the modifier
+# ':N' can be chained to compare the expression to several words or even
+# patterns in a sequence.  If one of the patterns matches, the final
+# expression will be the empty string.
+#
+.if ${:U word :None:Ntwo:Nthree} != ""
+#  good
+.else
+.  error
+.endif
+.if ${:U two :None:Ntwo:Nthree} != ""
+.  error
+.else
+#  good
+.endif
+#
+# The modifier ':N' is seldom used in general since positive matches with ':M'
+# are easier to grasp.  Chaining the ':N' modifier is even more difficult to
+# grasp due to the many negations involved.
+#
+# The final '!= ""' adds to the confusion because at first glance, the
+# condition may look like '${VAR} != ""', which for a single-word variable is
+# always true.
+#
+# The '!= ""' can be omitted if the expression cannot have the numeric value
+# 0, which is common in practice.  In that form, each ':N' can be pronounced
+# as 'neither' or 'nor', which makes the expression sound more natural.
+#
+.if ${:U word :None:Ntwo:Nthree}
+#  good
+.else
+.  error
+.endif
+.if ${:U two :None:Ntwo:Nthree}
+.  error
+.else
+#  good
+.endif
+#
+# Replacing the '${...} != ""' with '!empty(...)' doesn't improve the
+# situation as the '!' adds another level of negations, and the word 'empty'
+# is a negation on its own, thereby creating a triple negation.  Furthermore,
+# due to the '!empty', the expression to be evaluated no longer starts with
+# '$' and is thus more difficult to spot quickly.
+#
+.if !empty(:U word :None:Ntwo:Nthree)
+#  good
+.else
+.  error
+.endif
+.if !empty(:U two :None:Ntwo:Nthree)
+.  error
+.else
+#  good
+.endif
 
-# TODO: Implementation
 
 all:
-	@:;