Add OSX sed Does Regex A Bit Different 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).
 
-_1095 TILs and counting..._
+_1096 TILs and counting..._
 
 ---
 
@@ -948,6 +948,7 @@ _1095 TILs and counting..._
 - [Apply Multiple Substitutions To The Input](sed/apply-multiple-substitutions-to-the-input.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)
 - [Output Only Lines Involved In A Substitution](sed/output-only-lines-involved-in-a-substitution.md)
 - [Use An Alternative Delimiter In A Substitution](sed/use-an-alternative-delimiter-in-a-substitution.md)
 

sed/osx-sed-does-regex-a-bit-different.md

@@ -0,0 +1,32 @@
+# OSX sed Does Regex A Bit Different
+
+With GNU sed, `\+`, `\?`, `\(...\)` and friends are considered extended regex
+characters. You can use them directly with the preceding backslashes. Or you
+can include the `-r` flag to turn on extended regex and use them without.
+
+```bash
+$ echo '11+1 = 12' | sed 's/1+/3/'
+131 = 12
+$ echo '11+1 = 12' | sed -r 's/1+/3/'
+3+1 = 12
+```
+
+With OSX sed, `\+`, `\?`, and `\|` are not interpreted as part of the basic
+regex. To use them at all you need to include `-E` to turn on extended regex.
+The capture characters (`\(...\)`) are available with basic regex.
+
+```bash
+# Basic, always treated as literal +
+$ echo '11+1 = 12' | sed 's/1+/3/'
+131 = 12
+$ echo '11+1 = 12' | sed 's/1\+/3/'
+131 = 12
+
+# Extended, + is now a meta-character
+$ echo '11+1 = 12' | sed -E 's/1+/3/'
+3+1 = 12
+$ echo '11+1 = 12' | sed -E 's/1\+/3/'
+131 = 12
+```
+
+[source](https://unix.stackexchange.com/a/131940/5916)