I am looking for a regex substitution to transform N white spaces at the beginning of a line to N . So this text:
list:
- first
should become:
list:
- first
I have tried:
str = "list:\n - first"
str.gsub(/(?<=^) */, " ")
which returns:
list:
- first
which is missing one . How to improve the substitution to get the desired output?
You could make use of the \G anchor and \K to reset the starting point of the reported match.
To match all leading single spaces:
(?:\R\K|\G)
(?: Non capture group
\R\K Match a newline and clear the match buffer| Or\G Assert the position at the end of the previous match) Close non capture group and match a spaceSee a regex demo and a Ruby demo.
To match only the single leading spaces in the example string:
(?:^.*:\R|\G)\K
In parts, the pattern matches:
(?: Non capture group
^.*:\R Match a line that ends with : and match a newline| Or\G Assert the position at the end of the previous match, or at the start of the string) Close non capture group\K Forget what is matched so far and match a spaceSee a regex demo and a Ruby demo.
Example
re = /(?:^.*:\R|\G)\K /
str = 'list:
- first'
result = str.gsub(re, ' ')
puts result
Output
list:
- first
Use gsub with a callback function:
str = "list:\n - first"
output = str.gsub(/(?<=^|\n)[ ]+/) {|m| m.gsub(" ", " ") }
This prints:
list:
- first
The pattern (?<=^|\n)[ ]+ captures one or more spaces at the start of a line. This match then gets passed to the callback, which replaces each space, one at a time, with .