From 17057247a9521195d5b8f5ec4be8a3e4b68f3547 Mon Sep 17 00:00:00 2001
From: Michele Cereda <cereda.michele@gmail.com>
Date: Wed, 7 Sep 2022 13:20:01 +0200
Subject: [PATCH] Added SED character classes

---
 knowledge base/sed.md | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/knowledge base/sed.md b/knowledge base/sed.md
index 0a7321c..31a32a9 100644
--- a/knowledge base/sed.md	
+++ b/knowledge base/sed.md	
@@ -15,8 +15,27 @@ sed /etc/fstab \
   -e '/#.*\s*swap\s*.*/d'
 ```
 
+## Character classes and bracket expressions
+
+| Class | Description |
+| ----- | ----------- |
+| `[[:alnum:]]`  | alphanumeric characters `[[:alpha:]]` and `[[:digit:]]`; this is the same as `[0-9A-Za-z]` in the `C` locale and ASCII character |
+| `[[:alpha:]]`  | alphabetic characters `[[:lower:]]` and `[[:upper:]]`; this is the same as `[A-Za-z]` in the `C` locale and ASCII character encoding |
+| `[[:blank:]]`  | blank characters `space` and `tab` |
+| `[[:cntrl:]]`  | control characters; in ASCII these characters have octal codes 000 through 037 and 177 (DEL), in other character sets these are the equivalent characters, if any |
+| `[[:digit:]]`  | digits `0` to `9` |
+| `[[:graph:]]`  | graphical characters `[[:alnum:]]` and `[[:punct:]]` |
+| `[[:lower:]]`  | lower-case letters `a` to `z` in the `C` locale and ASCII character encoding |
+| `[[:print:]]`  | printable characters `[[:alnum:]]`, `[[:punct:]]` and `space` |
+| `[[:punct:]]`  | punctuation characters `!`, `"`, `#`, `$`, `%`, `&`, `'`, `(`, `)`, `*`, `+`, `,`, `-`, `.`, `/`, `:`, `;`, `<`, `=`, `>`, `?`, `@`, `[`, `\`, `]`, `^`, `_`, `` ` ``, `{`, `\|`, `}` and `~` in the `C` locale and ASCII character encoding |
+| `[[:space:]]`  | space characters `tab`, `newline`, `vertical tab`, `form feed`, `carriage return` and `space` in the `C` locale |
+| `[[:upper:]]`  | upper-case letters `A` to `Z` in the `C` locale and ASCII character encoding |
+| `[[:xdigit:]]` | hexadecimal digits `0` to `9`, `A` to `F` and `a` to `f` |
+
 ## Further readings
 
 - [GNU SED Online Tester]
+- [Character Classes and Bracket Expressions]
 
+[character classes and bracket expressions]: https://www.gnu.org/software/sed/manual/html_node/Character-Classes-and-Bracket-Expressions.html
 [gnu sed online tester]: https://sed.js.org/