Add OSX sed Does Regex A Bit Different as a sed til

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

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+# 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)