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=={{anchor|nullable symbol}}Formal theory==
Formally, a string is a finite, ordered sequence of [[character (symbol)|characters]] such as letters, digits or spaces. The empty string is the special case where the sequence has
There is only one empty string, because two strings are only different if they have different lengths or a different sequence of symbols.
In formal treatments,<ref>{{cite journal |first1=John |last1=Corcoran |first2=William |last2=Frank |first3=Michael |last3=Maloney |title=String theory |journal=Journal of Symbolic Logic |volume=39 |issue=4 |year=1974 |pages=625–637 |doi=10.2307/2272846 |jstor=2272846|s2cid=2168826 }}</ref> the empty string is denoted with '''[[ε]]''' or sometimes '''[[Λ]]''' or '''[[λ]]'''.
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