C# Regex Replace sequence of numbers preceded with a space

Question

I have this string:

Hello22, I'm 19 years old

I just want to replace the number with * if its preceded with a space, so it would look like this:

Hello22, I'm ** years old

I've been trying a bunch of regexes but no luck. Hope someone can help out with the correct regex. Thank you.

Regexes which I tried:

Regex.Replace(input, @"([\d-])", "*");

Returns all numbers replaced with *

Regex.Replace(input, @"(\x20[\d-])", "*");

Does not work as expected

Please include some of the regular expressions you have tried in your question. — ProgrammingLlama, Dec 11 '21 at 16:46
`(?<= )[0-9]+` or, may be, `\b[0-9]+\b` (here `\b` is a word break, that's why all `19` will match in `19, as I say, 19, I'm 19`) — Dmitry Bychenko, Dec 11 '21 at 16:49
@Alberto if Dmitry has the correct answer, you should mark it as accepted. — ProgrammingLlama, Dec 11 '21 at 17:30
Does this answer your question? [Reference - What does this regex mean?](https://stackoverflow.com/questions/22937618/reference-what-does-this-regex-mean) — AdrianHHH, Dec 12 '21 at 15:04

score 1 · Answer 1 · answered Dec 11 '21 at 16:56

You can try (?<= )[0-9]+ pattern where

(?<= ) - look behind for a space
[0-9]+ - one or more digits.

Code:

string source = "Hello22, I'm 19 years old";

string result = Regex.Replace(
  source, 
 "(?<= )[0-9]+", 
  m => new string('*', m.Value.Length));

Have a look at \b[0-9]+\b (here \b stands for word bound). This pattern will substitute all 19 in the "19, as I say, 19, I'm 19" (note, that 1st 19 doesn't have space before it):

string source = "19, as I say, 19, I'm 19";

string result = Regex.Replace(
   source, 
 @"\b[0-9]+\b", 
   m => new string('*', m.Value.Length));

score 0 · Answer 2 · answered Dec 11 '21 at 18:35

In C# you could also make use of a pattern with a lookbehind and an infinite quantifier.

(?<= [0-9]*)[0-9]

The pattern matches:

(?<= Positive lookbehind, assert what is to the left of the current position is
- [0-9]* Match a space followed by optional digits 0-9
) Close lookbehind
[0-9]\ Match a single digit 0-9

Example

string s = "Hello22, I'm 19 years old";
string result = Regex.Replace(s, "(?<= [0-9]*)[0-9]", "*");
Console.WriteLine(result);

Output

Hello22, I'm ** years old

C# Regex Replace sequence of numbers preceded with a space

2 Answers2