Add Equivalence Classes Of Repetition MetaChars as a sed til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1100 TILs and counting..._
+_1101 TILs and counting..._
 
 ---
 
@@ -948,6 +948,7 @@ _1100 TILs and counting..._
 ### sed
 
 - [Apply Multiple Substitutions To The Input](sed/apply-multiple-substitutions-to-the-input.md)
+- [Equivalence Classes Of Repetition MetaChars](sed/equivalence-classes-of-repetition-metachars.md)
 - [Extract Value From Command Output With Sed](sed/extract-value-from-command-output-with-sed.md)
 - [Grab All The Method Names Defined In A Ruby File](sed/grab-all-the-method-names-defined-in-a-ruby-file.md)
 - [OSX sed Does Regex A Bit Different](sed/osx-sed-does-regex-a-bit-different.md)

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

@@ -0,0 +1,45 @@
+# 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)