I want to replace all occurrences of a word in the whole buffer. What i do is:

M-x replace-string RET word-to-be-replaced RET new-word

The problem with this is that it also changes the word-to-be-replaced even if its only part of another word (for example wanting to change mod to off_mod results also changing set_mode to set_off_mode). How can i change it only in cases it exists exactly as a whole word and not also as parts of other words?

  • Have a look at regular expressions.
    – Dan
    May 14, 2016 at 22:51

1 Answer 1


If you provide the universal argument to replace-string, that is by pressing C-u before M-x replace-string, it only replaces matches that are surrounded by word boundaries.

So, for example, a buffer containing


would be become after C-u M-x replace-string RET mod RET off_mod


You can find this information by looking in the manual: C-h i (to get the info index) m Emacs RET (to open the Emacs user manual section) and finding "replace-string" through the topic index by doing i replace-string RET.

Alternatively, you could look-up the function's documentation by doing C-h f replace-string

  • 6
    +1, especially for teaching about the manual. Also: C-h r takes you to the Emacs manual, where i replace-string takes you directly to the doc about replace-string. i is your friend in Info.
    – Drew
    May 15, 2016 at 0:21

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