Linux Editor that can highlight all instances of a selected word

linux editor syntax-highlighting

To enable this feature in Geany do the following:

Install the addons plugin: sudo apt-get install geany-plugin-addons
In Geany, click on Tools -> Plugin Manager
enable the plugin Addons (check in the column Active)
configure the plugin by clicking on Preferences
enable the option Mark all occurrences of a word when double-clicking it

linux editor syntax-highlighting

First install the addons plugin:

sudo apt-get install geany-plugin-addons

and then apply the answer of mr13.

linux editor syntax-highlighting

What version of Kate where you using, Kate has that exact feature as described here.