Regex to insert space in vim

Question

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 inst

Answer 1

:%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.