A partial word is a string that may contain a number of "do not know" or "do not care" symbols i.e. placeholders in the string where the symbol value is not known or not specified. More formally, a partial word is a partial function
u
:
{
0
,
…
,
n
−
1
}
→
A
where
A
is some finite alphabet. If u(k) is not defined for some
k
∈
{
0
,
…
,
n
−
1
}
then the unknown element at place k in the string is called a "hole". In regular expressions (following the POSIX standard) a hole is represented by the metacharacter ".". For example, aab.ab.b is a partial word of length 8 over the alphabet A ={a,b} in which the fourth and seventh characters are holes.
