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46 lines
1017 B
Markdown
46 lines
1017 B
Markdown
# Equivalence Classes Of Repetition MetaChars
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There are two types of Repetition MetaChars. The simple ones are `*`, `+`, and
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`?`. The general ones are ranges specified inside `{` and `}`. Here are
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equivalence classes between these two sets.
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_These use the -E (extended regex) option for OSX's `sed`._
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1. `*` is equivalent to `{0,}`
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_Zero or more of the preceeding character._
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```bash
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$ echo 'abc123' | sed -E 's/[[:alpha:]]*/!/'
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!123
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$ echo 'abc123' | sed -E 's/[[:alpha:]]{0,}/!/'
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!123
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```
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2. `+` is equivalent to `{1,}`
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_One or more of the preceeding character._
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```bash
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$ echo 'fix the spacing' | sed -E 's/[ ]+/ /g'
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fix the spacing
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$ echo 'fix the spacing' | sed -E 's/[ ]{1,}/ /g'
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fix the spacing
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```
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3. `?` is equivalent to `{0,1}`
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_Exactly zero or one of the preceeding character._
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```bash
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$ echo '#1, 2, 1oz' | sed -E 's/#?1/ONE/g'
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ONE, 2, ONEoz
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$ echo '#1, 2, 1oz' | sed -E 's/#{0,1}1/ONE/g'
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ONE, 2, ONEoz
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```
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[source](https://www.goodreads.com/book/show/19407377-definitive-guide-to-sed)
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