In Intellij, how do I toggle between camel case and underscore spaced?

Very simple press Clr + F to open Find/Replace panel and check [✓] Regex copy past regex

Find: [_]{1,1}([a-z])

Replace: \U$1

Press [Replace all] button, Enjoy

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Thanks @piotrek for _some_awe_var to _someAweVar

Use Find: (\w)[_]{1,1}([a-z])
Replace: $1\U$2

From snake_case to CamelCase

• Find: (\w)[_]{1,1}([a-z])
• Replace: $1\U$2
• Settings:
  • Match Case
  • Regex

From CamelCase to snake_case:

• Find: ([A-Z])
• Replace: \_\L$1
• Settings:
  • Match Case
  • Regex

If you are OK with PyCharm also refactoring usages, launch the "Rename" tool (Refactor > Rename). The window pops up with drop down list and you should see the snake_case version of the text in the list (you can open the window and switch to the snake_case with key-strokes so it can be pretty fast).

Two plugins offer this feature:

I use a plugin called String Manipulation which does what you need (and more).

Select historyOfPresentIllness and press Alt / option+M to bring up the plugin menu, then press:

• 5 - To snake_case (or to camelCase) which converts to history_of_present_illness
• 6 - To hyphen-case (or to snake_case) which converts to history-of-present-illness

To make this easier, you could set up a shortcut at File | Settings | Keymap.

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There also is the CamelCase plugin.

SHIFT+Alt / option+U toggles the selection between formats:

historyOfPresentIllness --> history_of_present_illness --> HISTORY_OF_PRESENT_ILLNESS --> HistoryOfPresentIllness --> historyOfPresentIllness

You can also undo your changes (now that a bug in the plugin got fixed).