Add Equivalence Classes Of Repetition MetaChars as a sed til

sed/equivalence-classes-of-repetition-metachars.md

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+# 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._
+
+```bash
+$ 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._
+
+```bash
+$ 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._
+
+```bash
+$ 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
+```
+
+[source](https://www.goodreads.com/book/show/19407377-definitive-guide-to-sed)