How to insert spaces between characters using Regex?

Regex.Replace("Hello", "(.)", "$1 ").TrimEnd();

Explanation

The dot character class matches every character of your string "Hello".
The paranthesis around the dot character are required so that we could refer to the captured character through the $n notation.
Each captured character is replaced by the replacement string. Our replacement string is "$1 " (notice the space at the end). Here $1 represents the first captured group in the input, therefore our replacement string will replace each character by that character plus one space.
This technique will add one space after the final character "o" as well, so we call TrimEnd() to remove that.

A demo can be seen here.

For the enthusiast, the same effect can be achieve through LINQ using this one-liner:

String.Join(" ", YourString.AsEnumerable())

or if you don't want to use the extension method:

String.Join(" ", YourString.ToCharArray())

You could do this through regex only, no need for inbuilt c# functions. Use the below regexes and then replace the matched boundaries with space.

(?<=.)(?!$)

DEMO

string result = Regex.Replace(yourString, @"(?<=.)(?!$)", " ");

Explanation:

(?<=.) Positive lookbehind asserts that the match must be preceded by a character.
(?!$) Negative lookahead which asserts that the match won't be followed by an end of the line anchor. So the boundaries next to all the characters would be matched but not the one which was next to the last character.

You could also use word boundaries.

(?<!^)(\B|b)(?!$)

DEMO

string result = Regex.Replace(yourString, @"(?<!^)(\B|b)(?!$)", " ");

Explanation:

(?<!^) Negative lookbehind which asserts that the match won't be at the start.
(\B|\b) Matches the boundary which exists between two word characters and two non-word characters (\B) or match the boundary which exists between a word character and a non-word character (\b).
(?!$) Negative lookahead asserts that the match won't be followed by an end of the line anchor.

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