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(/(?<=^) */, "&nbsp;")

which returns:

list:
 - first

which is missing one  . How to improve the substitution to get the desired output?

CodePudding user response：

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 space

See 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 space

See a regex demo and a Ruby demo.

Example

re = /(?:^.*:\R|\G)\K /
str = 'list:
  - first'
result = str.gsub(re, ' ')

puts result

Output

list:
  - first

CodePudding user response：

Use gsub with a callback function:

str = "list:\n  - first"
output = str.gsub(/(?<=^|\n)[ ] /) {|m| m.gsub(" ", "&nbsp;") }

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  .