Wildcards

Another special case of a word is a wildcard.

$<$wildcard$>$:==

{ $*$ $\vert$ $\%$ $\vert$ $\char93$ $\vert$ $+$ $\vert$ $<$string$>$ }

$*$ matches any string.

$\%$ matches a single character.

$\char93$ matches any number.

$+$ matches any digit.

It is used to match a construct against sets of other words. The possibility of a wildcard specification is also indicated by enclosing syntactic constructs in $<$* *$>$ angle brackets.