How do I substitute with a character from the matched text, where the pattern contains a range?

Question

If I'm using a range in a regex, how can I replace with the character that was matched? For example, lets say I want to find all occurrences of a or b, and replace them with aa or bb. Can I do this in a single regex? For example, something like this:

:%s/[a-b]/\1\1/g

Where \1 means the first matching character? Can I extend this to several characters? For example converting camelCase to snake_case. Can I do something like this:

%s/[a-z][A-Z]/\1_\2/g

Where \1 is the first matching character and \2 is the second matching character?

Answer 1

For using the entire matched text, just use &:

magic   nomagic   action    
  &       \&      replaced with the whole matched pattern            s/\&
 \&        &      replaced with &

So:

%s/[a-b]/&&/g

For using the matched text in parts, use regex groups:

\(\)    A pattern enclosed by escaped parentheses.      /\( /\(\) /\)
        E.g., "\(^a\)" matches 'a' at the start of a line.
        E51 E54 E55 E872 E873

\1      Matches the same string that was matched by     /\1 E65
        the first sub-expression in \( and \). {not in Vi}
        Example: "\([a-z]\).\1" matches "ata", "ehe", "tot", etc.
\2      Like "\1", but uses second sub-expression,      /\2
   ...                                                  /\3
\9      Like "\1", but uses ninth sub-expression.       /\9
        Note: The numbering of groups is done based on which "\(" comes first
        in the pattern (going left to right), NOT based on what is matched
        first.

So:

%s/\([a-z]\)\([A-Z]\)/\1_\2/g

Or more conveniently, enabling magic:

%s/\v([a-z])([A-Z])/\1_\2/g

Answer 2

You can easily change from camelCase style to snake_case style via the Abolish.vim plugin

%s/\<\l\+\u\w\+/\=Abolish.snakecase(submatch(0))/g

Abolish also provides "Coerce" mappings:

• crc - coerce to camelCase
• crs - coerce to snake_case
• crm - coerce to MixCased
• cru - coerce to UPPERCASE_SNAKE_CASE
• cr- - coerce to dashed-case

There is Vimcasts eposide on use Abolish: Smart search with :Subvert

For more help see:

:h /\<
:h /\l
:h /\u
:h sub-replace-expression