Regular Expression to add space to each dash at the beginning of string

var input = "---Three dashes a-b-c-d";

Regex to get

var output = "- - - Three dashes a-b-c-d"; 

and Regex get back to original, added spaces are deleted at the beginning. output => input

Code without Regex input => output

var input = "---Three dashes a-b-c-d";
var output = "";
var i = 0;
while (i < input.Length && input[i] == '-')
{
    output += "- ";
    i++;
}
output += input.Substring(i);

While @TedLyngmo's solution works, it is rather inefficient with a wild card in the lookbehind pattern.

A more efficient approach would be to use a positive lookbehind pattern to assert that only dashes precede a match:

@"(?<=^-+)"

And substitute the match with a space.

On regex101, the above regex took 0.2ms to run while @TedLyngmo's took 19.5ms.

Demo: https://regex101.com/r/6uy1K8/3

Regex (with a trailing space) to remove the added spaces to get back the original:

@"(?<=^(?:- )*-) "

^{Note the space at the end}

And substitute the match with nothing.

On regex101, the above regex took 0.3ms to run while @TedLyngmo's took 28.1ms.

Demo: https://regex101.com/r/fBOirM/3

Note that the desired behaviors require a regex engine that supports variable-width lookbehind, which C# happens to have. The above regexes are otherwise not portable to platforms that do not have such a support.

One way could be a negative lookbehind to check that no character before a dash to add a space too is anything but a dash:

@"(?<![^-].*)-"

• (?<! - start of negative lookbehind
  • [^-] - any character but -
  • .* - zero or more of any character
• ) - end of negative lookbehind
• - - a literal -

Match globally and substitute with - (dash + space)

Demo

Removing the added spaces to get the original back could be done in a similar way:

@"(?<![^- ].*)- "

Match globally and substitute with - (dash).

Demo

In C# you can use the \G anchor to assert the position at the end of the previous match or at the start of the string:

\G-

In the replacement use - or you can use the full match followed by a space like $0

See a .NET regex demo and a C# demo.

var input = "---Three dashes a-b-c-d";
string result = Regex.Replace(input, @"\G-", "- ");
Console.WriteLine(result);

Output

- - - Three dashes a-b-c-d

<hr>

If you want to change it back to the original, you can add matching the space again:

\G-

In the replacement use just -

See another .NET regex demo.

var input = "- - - Three dashes a-b-c-d";
string result = Regex.Replace(input, @"\G- ", "-");
Console.WriteLine(result);

Output

---Three dashes a-b-c-d