Replacing or searching text with regular expressions is very common. But sometimes it is hard to get them right and you need some playground to try them out. In order to verify regular expressions i used to visit webpages like:

But these webpages are optimised for perl, javascript or bash expressions and do not support Emacs regular expressions directly. In Emacs you have to consider some peculiarities. For example you have to escape parentheses.

Recently i learnt about M-x regexp-builder with which you can test your regular-expressions interactively. Just enter the regular expression and the text in your buffer will be highlighted immediately. It even considers different capture groups and highlights them in different colours. Very nice!