Regex: Match everything until the last white space

Question

I want to reduce the length of my input string to max 20 characters, but I don't want't to break the string in the middle of a word.

// show me 20 char:   12345678901234567890 
string inputString = "This is an example user input which has to be shorten at a white space";
if (inputString.length > 20)
{
    shortenString = inputString.SubString(0, 21); // <-- "This is an example us"
    
    // I need a regex to match everything until the last white space

    // final output: "This is an example"
}

`var output = inputString.Split(' ').Aggregate(new List() { "" }, (a, x) => { if (a.Last().Length + x.Length + 1 < 20) { a[a.Count - 1] = $"{a.Last()} {x}".Trim(); } else { a.Add(x); } return a; });` — Enigmativity, Jun 23 '20 at 01:20
```(.{0,20})\s``` this regex will capture groups up to 20 characters, ending in whitespace — Ced, Jun 23 '20 at 01:22
@Enigmativity .{0,20} will match between 0 and 20 of the preceding character, then \s will match a whitespace. — Ced, Jun 23 '20 at 01:25
`var result = shortenString.Contains(" ") ? shortenString.Substring(0, shortenString.LastIndexOf(" ")) : shortenString;` is one line. — Rufus L, Jun 23 '20 at 01:29
@Enigmativity the capturing group, designated by the ( ), will limit to 20 characters. — Ced, Jun 23 '20 at 01:34
@Ced - It would also need to cover strings that don't end in a whitespace and are shorter than 20 characters. — Enigmativity, Jun 23 '20 at 01:34
@Enigmativity why don't you re-open the question, seeing as it clearly isn't a duplicate, then OP can clarify whether my answer is acceptable. Seeing as he's the one asking the question, I'd rather take his opinion on it than yours — Ced, Jun 23 '20 at 01:38
@HoomanBahreini careful with that regex you linked, it will not limit to 20 characters, but capture everything up to the last whitespace — Ced, Jun 23 '20 at 02:09
@HoomanBahreini - If you have an answer please post it. However, the link you posted doesn't answer the question you've asked. — Enigmativity, Jun 23 '20 at 02:17

Ced · Accepted Answer · 2020-06-23T02:34:29.103

2

(.{0,20})(\s|$) this regex will capture groups up to 20 characters, ending in whitespace (or end of string)

edited Jun 23 '20 at 02:34

answered Jun 23 '20 at 01:53

Ced

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@Enigmativity correct. I've updated it to also match end of string, which will now capture "Hello world" – Ced Jun 23 '20 at 02:40
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@Ced: Thanks for your help, I used your original regex: `^(.{0,20})\s` with a minor change (added `^` to ensure it counts from beginning of string)... I didn't use the end anchor `$`... instead added an if condition to ignore cases where input is too short. – Hooman Bahreini Jun 23 '20 at 02:56

Hooman Bahreini · Answer 2 · 2020-06-23T03:14:48.473

1

I used the RegEx from @Ced answer, this is the final extension method:

public static string ShortenStringAtWhiteSpaceIfTooLong(this string str, int maxLen)
{
    if (str.Length <= maxLen)
    {
        return str;
    }

    string pattern = @"^(.{0," + maxLen.ToString() + @"})\s"; // <-- @"^(.{0,20})\s" 
    Regex regEx = new Regex(pattern);
    var matches = regEx.Matches(str);

    if (matches.Count > 0)
    {
        return matches[0].Value.Trim();
    }
    else
    {
        return str.Substring(0, maxLen).Trim();
    }
}

edited Jun 23 '20 at 03:14

answered Jun 23 '20 at 02:53

Hooman Bahreini

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I might suggest that `string pattern = $@"^(.{{0,{maxLen}}})\s";` is cleaner. – Enigmativity Jun 23 '20 at 05:23

Regex: Match everything until the last white space

2 Answers2