equivalence-classes-of-repetition-metachars.md

Equivalence Classes Of Repetition MetaChars

There are two types of Repetition MetaChars. The simple ones are *, +, and ?. The general ones are ranges specified inside { and }. Here are equivalence classes between these two sets.

These use the -E (extended regex) option for OSX's sed.

1. * is equivalent to {0,}

Zero or more of the preceeding character.

$ echo 'abc123' | sed -E 's/[[:alpha:]]*/!/'
!123

$ echo 'abc123' | sed -E 's/[[:alpha:]]{0,}/!/'
!123

2. + is equivalent to {1,}

One or more of the preceeding character.

$ echo 'fix   the spacing' | sed -E 's/[ ]+/ /g'
fix the spacing

$ echo 'fix   the spacing' | sed -E 's/[ ]{1,}/ /g'
fix the spacing

3. ? is equivalent to {0,1}

Exactly zero or one of the preceeding character.

$ echo '#1, 2, 1oz' | sed -E 's/#?1/ONE/g'
ONE, 2, ONEoz

$ echo '#1, 2, 1oz' | sed -E 's/#{0,1}1/ONE/g'
ONE, 2, ONEoz

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