The fstclosure command implements Kleene closure/Kleene star. That is, it converts a set of strings into the set of strings consisting of zero or more repetitions of strings in the input set. The command can also be used to emulate the “+” operator with --closure_plus flag.

For example, given a simple automaton representing the regular expression a(b|c):

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Initial automaton
fstprint --osymbols=words.txt --isymbols=words.txt L.fst
0 1 a a
1 2 b b
1 3 c c
2 4 <eps> <eps>
3 4 <eps> <eps>
4

The language a(b|c)

Running fstclosure produces (a(b|c))*:

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Run fstclosure for Kleene Star
fstclosure L.fst Lstar.fst
fstprint --osymbols=words.txt --isymbols=words.txt Lstar.fst
5 0 <eps> <eps>
5
0 1 a a
1 2 b b
1 3 c c
2 4 <eps> <eps>
3 4 <eps> <eps>
4 0 <eps> <eps>
4
========================

The language (a(b|c))*

While running fstclosure --closure_plus produces (a(b|c))+.

========================
Run fstclosure for Kleene plus
fstclosure --closure_plus L.fst Lplus.fst
fstprint --osymbols=words.txt --isymbols=words.txt Lplus.fst
0 1 a a
1 2 b b
1 3 c c
2 4 <eps> <eps>
3 4 <eps> <eps>
4 0 <eps> <eps>
4

The language (a(b|c))+