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Asked: 2026-05-27T09:06:04+00:00

I am a regex supernoob (just reading my first articles about them), and at

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I am a regex supernoob (just reading my first articles about them), and at the same time working towards stronger use of vim. I would like to use a regex to search for all instances of a colon : that are not followed by a space and insert one space between those colons and any character after them.

If I start with:

foo:bar

I would like to end with

foo: bar

I got as far as %s/:[a-z] but now I don’t know what do for the next part of the %s statement.

Also, how do I change the :[a-z] statement to make sure it catches anything that is not a space?

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1 Answer

  1. Editorial Team

    :%s/:\(\S\)/: \1/g

    \S matches any character that is not whitespace, but you need to remember what that non-whitespace character is. This is what the \(\) does. You can then refer to it using \1 in the replacement.

    So you match a :, some non-whitespace character and then replace it with a :, a space, and the captured character.

    Changing this to only modify the text when there’s only one : is fairly straight forward. As others have suggested, using some of the zero-width assertions will be useful.

    :%s/:\@!<:[^:[:space:]]\@=/: /g

    • :\@!< matches any non-:, including the start of the line. This is an important characteristic of the negative lookahead/lookbehind assertions. It’s not requiring that there actually be a character, just that there isn’t a :.

    • : matches the required colon.

    • [^:[:space:]] introduces a couple more regex concepts.

      • The outer [] is a collection. A collection is used to match any of the characters listed inside. However, a leading ^ negates that match. So, [abc123] will match a, b, c, 1, 2, or 3, but [^abc123] matches anything but those characters.

      • [:space:] is a character class. Character classes can only be used inside a collection. [:space:] means, unsurprisingly, any whitespace. In most implementations, it relates directly to the result of the C library’s isspace function.

      Tying that all together, the collection means “match any character that is not a : or whitespace”.

    • \@= is the positive lookahead assertion. It applies to the previous atom (in this case the collection) and means that the collection is required for the pattern to be a successful match, but will not be part of the text that is replaced.

    So, whenever the pattern matches, we just replace the : with itself and a space.

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