Removing commas from numbers with .NET regex

So I'm processing a report that (brilliantly, really) spits out number values with commas in them, in a .csv output. Super useful.

So, I'm using (C#)regex lookahead positive and lookbehind positive expressions to remove commas that have digits on both sides.

If I use only the lookahead, it seems to work. However when I add the lookbehind as well, the expression breaks down and removes nothing. Both ends of the comma can have arbitrary numbers of digits around them, so I just want to remove the comma if the pattern has one or more digits around it.

Here's the expression that works with the lookahead only:

str = Regex.Replace(str, @"[,](?=(d+)),"");


Here's the expression that doesn't work as I intend it:

str = Regex.Replace(str, @"[,](?=(d+)?<=(d+))", "");


What's wrong with my regex! If I had to guess, there's something I'm misunderstanding about how lookbehind works. Any ideas?

Regex.Replace(str, @"(d),(d)", "$1$2")

1,2,3,4

1, 2, 3, 4

1.2, 3.4

1, 2.3, 4

Regex.Replace(s, @"(?<=d),(?=d)", string.Empty)

2 Answers
2

You can always check a website that evaluates regex expressions.
I think this code might be able to help you:

str = Regex.Replace(str, @"[,](?=(d+))(?<=(d))", "");


You may use any of the solutions below:

var s = "abc,def,2,100,xyz!,:))))";
Console.WriteLine(Regex.Replace(s, @"(d),(d)", "$1$2")); // Does not handle 1,2,3,4 cases
Console.WriteLine(Regex.Replace(s, @"(d),(?=d)", "$1")); // Handles consecutive matches with capturing group+backreference/lookahead
Console.WriteLine(Regex.Replace(s, @"(?<=d),(?=d)", "")); // Handles consecutive matches with lookbehind/lookahead, the most efficient way
Console.WriteLine(Regex.Replace(s, @",(?<=d,)(?=d)", "")); // Also handles all cases


See the C# demo.

Explanations:

(d),(d)

,

$1$2

(d),(?=d)

,

(?=d)

,

$1

(?<=d),(?=d)

(?<=d)

,(?<=d,)(?=d)

RegexHero.net test:

Bonus:

You may just match a pattern like yours with d,d and pass the match to the MatchEvaluator method where you may manipulate the match further:

d,d

MatchEvaluator

Console.WriteLine(Regex.Replace(s, @"d,d", m => m.Value.Replace(",",string.Empty))); // Callback method


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