Replace string if starts with string in List

Question

I have a string that looks like this

s = "<Hello it´s me, <Hi  how are you <hay"

and a List List<string> ValidList= {Hello, hay} I need the result string to be like

string result = "<Hello it´s me, ?Hi  how are you <hay"

So the result string will if it starts with an < and the rest bellogs to the list, keep it, otherwise if starts with < but doesn´t bellong to list replaces the H by ?

I tried using the IndexOf to find the position of the < and the if the string after starsWith any of the strings in the List leave it.

foreach (var vl in ValidList)
{
    int nextLt = 0;
    while ((nextLt = strAux.IndexOf('<', nextLt)) != -1)
    {

        //is element, leave it
        if (!(strAux.Substring(nextLt + 1).StartsWith(vl)))
        {
            //its not, replace
            strAux = string.Format(@"{0}?{1}", strAux.Substring(0, nextLt), strAux.Substring(nextLt + 1, strAux.Length - (nextLt + 1)));
        }
       nextLt++;   
    }
}

Regex.Replace(s, string.Format("<(?!{0})", string.Join("|", ValidList)), "?") — poke
– poke, Commented Aug 1, 2017 at 11:31
but in the case that has this string s = "<Hello it´s me < d" it doesn´t replace the <, that is alone — hi itsme
– hi itsme, Commented Aug 1, 2017 at 11:35
You didn’t specify at all what should happen with < that are not a prefix to a word. Nor did you give an example. Nor did you say anything about all the other edge cases I can think about. — poke
– poke, Commented Aug 1, 2017 at 11:37
@hiitsme Solution given @ poke will work perfectly fine for your case, check this online — mrogal.ski
– mrogal.ski, Commented Aug 1, 2017 at 11:40

poke · Accepted Answer · 2017-08-01 11:51:30Z

2

To give the solution I gave as a comment its proper answer:

Regex.Replace(s, string.Format("<(?!{0})", string.Join("|", ValidList)), "?")

This (obviously) uses regular expressions to replace the unwanted < characters by ?. In order to recognize those characters, we use a negative lookahead expression. For the example word list, this would look like this: (?!Hallo|hay). This will essentially match only if what we are matching is not followed by Hallo or hay. In this case, we are matching < so the full expression becomes <(?!Hallo|hay).

Now we just need to account for the dynamic ValidList by creating the regular expression on the fly. We use string.Format and string.Join there.

answered Aug 1, 2017 at 11:51

poke

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Comments

Rahul · Accepted Answer · 2017-08-01 12:25:26Z

Something like this without using RegEx or LINQ

        string s = "<Hello it´s me, <Hi  how are you <hay";
        List<string> ValidList = new List<string>() { "Hello", "hay" };

        var arr = s.Split(new[] { '<' }, StringSplitOptions.RemoveEmptyEntries);
        for (int i = 0; i < arr.Length; i++)
        {
            bool flag = false;
            foreach (var item in ValidList)
            {
                if (arr[i].Contains(item))
                {
                    flag = false;
                    break;
                }
                else
                {
                    flag = (flag) ? flag : !flag;
                }
            }

            if (flag)
                arr[i] = "?" + arr[i];
            else
                arr[i] = "<" + arr[i];
        }

        Console.WriteLine(string.Concat(arr));

Pikoh · Accepted Answer · 2017-08-01 11:44:27Z

0

A possible solution using LINQ.It splits the string using < and checks if the "word" (text until a blank space found) following is in the Valid List,adding < or ? accordingly. Finally,it joins it all:

List<string> ValidList = new List<string>{ "Hello", "hay" };
string str = "<Hello it´s me, <Hi  how are you <hay";

var res = String.Join("",str.Split(new char[] { '<' }, StringSplitOptions.RemoveEmptyEntries)
                 .Select(x => ValidList.Contains(x.Split(' ').First()) ? "<" + x : "?"+x));

answered Aug 1, 2017 at 11:44

Pikoh

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