You want to search and replace content in your posts, pages or comments?
A very straight forward plugin to do this is the Search Regex Plugin
You can search for simple strings or regular expressions and you can preview the replace operation before applying it (replace & save button).
Compared to raw regular expressions as used for example in the online regular expression tester Regexpal, you must enclose your expression with @ characerts (can be changed in the configuration).
In case, you need to move your blog and replace all internal links, the plugin can be very handy as well!
Plugin Website: https://wordpress.org/plugins/search-regex/