Create regex pattern containing parentheses

Question

I'm using a regexpattern to highlight a text matching a given search criteria in WPF. When the user types in San, my function creates a pattern as (San). The effect in my Application is that the search result display all hits containg "San" with the phrase "San" highlighted. However, if the search string contatins parentheses or backslash such as "San("

creating the regex crashes With 'too many or too few '(' or similar, as the pattern would be (San()

Creating the pattern With the Regex.Escape solves the crash, but it also Escapes the initial parentheses. I could manipulate the pattern string after creating the regex, but is there a more streamlined way to do this?

private IEnumerable<Inline> CreateInlines(string text, string[] highlights)
    {
        var pattern = string.Join("|", highlights
            .Where(highlight => !string.IsNullOrEmpty(highlight))
            .Select(highlight => "(" + highlight + ")")
            .ToArray());

        if (string.IsNullOrEmpty(pattern))
        {
            yield return new Run(text);
            yield break;
        }

        var regex = new Regex(Regex.Escape(pattern), RegexOptions.IgnoreCase);
        var matches = regex.Matches(text);

        if (matches.Count < 1)
        {
            yield return new Run(text);
            yield break;
        }

        int offset = 0;
        for (int i = 0; i < matches.Count; i++)
        {
            var match = matches[i];
            if (match.Index > offset)
            {
                int length = match.Index - offset;
                yield return new Run(text.Substring(offset, length));
                offset += length;
            }

            if (match.Length > 0)
            {
                yield return new Run(text.Substring(offset, match.Length)) { Foreground = HighlightBrush };
                offset += match.Length;
            }
        }

        if (offset < text.Length)
        {
            yield return new Run(text.Substring(offset));
        }
    }

The answer is: do not enclose the pattern with parentheses. Another point: you most probably can reduce this greatly if you just use Regex.Replace — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Jan 11, 2018 at 8:06

Sweeper · Accepted Answer · 2018-01-11 08:00:17Z

1

Do the Regex.Escape here:

    var pattern = string.Join("|", highlights
        .Where(highlight => !string.IsNullOrEmpty(highlight))
        .Select(highlight => "(" + Regex.Escape(highlight) + ")")
        .ToArray());

Also, C# 6 has string interpolation:

    var pattern = string.Join("|", highlights
        .Where(highlight => !string.IsNullOrEmpty(highlight))
        .Select(highlight => $"({Regex.Escape(highlight)})")
        .ToArray());

In fact, you don't need the parentheses at all! You are not capturing anything (at least I don't see you use Groups in your code).

answered Jan 11, 2018 at 8:00

Sweeper

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1 Comment

lewi Over a year ago

Correct, the parentheses was remains from when use of groups was intended :)

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