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Expressions as Patterns

   Any `awk' expression is also valid as an `awk' pattern.  Then the
pattern "matches" if the expression's value is nonzero (if a number) or
nonnull (if a string).

   The expression is reevaluated each time the rule is tested against a
new input record.  If the expression uses fields such as `$1', the
value depends directly on the new input record's text; otherwise, it
depends only on what has happened so far in the execution of the `awk'
program, but that may still be useful.

   Comparison patterns are actually a special case of this.  For
example, the expression `$5 == "foo"' has the value 1 when the value of
`$5' equals `"foo"', and 0 otherwise; therefore, this expression as a
pattern matches when the two values are equal.

   Boolean patterns are also special cases of expression patterns.

   A constant regexp as a pattern is also a special case of an
expression pattern.  `/foo/' as an expression has the value 1 if `foo'
appears in the current input record; thus, as a pattern, `/foo/'
matches any record containing `foo'.

   Other implementations of `awk' that are not yet POSIX compliant are
less general than `gawk': they allow comparison expressions, and
boolean combinations thereof (optionally with parentheses), but not
necessarily other kinds of expressions.