It's just part of Markdown - Markdown provides backslash escapes for the following characters: \ backslash ` backtick * asterisk _ underscore {} curly braces [] square brackets () parentheses # hash mark + plus sign - minus sign (hyphen) . dot ! exclamation mark
You can also specify a range of characters using -inside square brackets. [a-e] is the same as [abcde]. [1-4] is the same as [1234]. [0-39] is the same as [01239]. You can complement (invert) the character set by using caret ^ symbol at the start of a square-bracket. [^abc] means any character except a or b or c. [^0-9] means any non-digit ...