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Star height : ウィキペディア英語版
Star height
In theoretical computer science, more precisely in the theory of formal languages, the star height is a measure for the structural complexity
of regular expressions: The star height equals the maximum nesting depth of stars appearing in the regular expression.
The concept of star height was first defined and studied by Eggan (1963).
==Formal definition==
More formally, the star height of a regular expression
''E'' over a finite alphabet ''A'' is inductively defined as follows:
* \scriptstyle h\left(\emptyset\right)\,=\,0, \scriptstyle h\left(\varepsilon\right)\,=\,0, and \scriptstyle h\left(a\right)\,=\,0 for all alphabet symbols ''a'' in ''A''.
* \scriptstyle h\left(E F\right)\,=\, h\left(E\, \mid\, F\right)\,=\,\max \left(\, h(E), h(F)\,\right)
* \scriptstyle h\left(E^
*\right)\,=\,h(E)+1.
Here, \scriptstyle \emptyset is the special regular expression denoting the empty set and ε the special one denoting the empty word;
''E'' and ''F'' are arbitrary regular expressions.
The star height ''h''(''L'') of a regular language ''L'' is defined as the minimum star height among all regular expressions representing ''L''.
The intuition is here that if the language ''L'' has large star height, then it is in some sense inherently complex, since it cannot be described
by means of an "easy" regular expression, of low star height.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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