Regular expressions is used to search in text.
Standard character classes
- A character class is created by putting things inside a 
- So gr[ae]y match both gray and grey
- Use a - inside to specify a range. [0-9] for a single digit between 0 and 9.
- It's possible to specify more then one range and single characters. Ex [0-9a-fq] is 0-9, a-f or q.
- Use a caret after the opening square to negate the class. [^0-9] will match any non number character.
Standard Character set
- Some character sets is so common that shorthand names already exist for them
- \d match a single character digit
- \w Match alphanumeric characters and _.
- \s Match white space characters.
Search for %x and capture the word behind it.
%xx (.*?) %xx
Capture the text between %xx and %xx. To use the capture text in a search and replace use \1 where the number is the index of the capture group. So use <b>\1</b> as the replace text to replace %xx Hello world %xx with <b>Hello World</b>.
At start of line (^) find @title, skip any number of spaces behind it (\s+) and then capture the rest of the text (.*).
At start of line capture any number of # (#+). Skip space and then capture the rest of the text.