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PORegular expression for String.Format-like utility
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  1. PORegular expression for String.Format-like utility
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    copied!<p>I'm writing a class called <code>StringTemplate</code>, which allows to format objects like with <code>String.Format</code>, but with names instead of indexes for placeholders. Here's an example :</p> <pre><code>string s = StringTemplate.Format("Hello {Name}. Today is {Date:D}, and it is {Date:T}.", new { Name = "World", Date = DateTime.Now }); </code></pre> <p>To achieve this result, I look for placeholders and replace them with indexes. I then pass the resulting format string to <code>String.Format</code>.</p> <p>This works fine, except when there are doubled braces, which are an escape sequence. The desired behavior (which is the same as <code>String.Format</code>) is described below :</p> <ul> <li><em>"Hello {Name}"</em> should be formatted as <em>"Hello World"</em></li> <li><em>"Hello {{Name}}"</em> should be formatted as <em>"Hello {Name}"</em></li> <li><em>"Hello {{{Name}}}"</em> should be formatted as <em>"Hello {World}"</em></li> <li><em>"Hello {{{{Name}}}}"</em> should be formatted as <em>"Hello {{Name}}"</em></li> </ul> <p>And so on...</p> <p>But my current regular expression doesn't detect the escape sequence, and always considers the substring between brackets as a placeholder, so I get things like <em>"Hello {0}"</em>.</p> <p>Here's my current regular expression :</p> <pre><code>private static Regex _regex = new Regex(@"{(?&lt;key&gt;\w+)(?&lt;format&gt;:[^}]+)?}", RegexOptions.Compiled); </code></pre> <p><strong>How can I modify this regular expression to ignore escaped braces ?</strong> What seems really hard is that I should detect placeholders depending on whether the number of brackets is odd or even... I can't think of a simple way to do it with a regular expression, is it even possible ?</p> <hr> <p>For completeness, here's the full code of the <code>StringTemplate</code> class :</p> <pre><code>public class StringTemplate { private string _template; private static Regex _regex = new Regex(@"{(?&lt;key&gt;\w+)(?&lt;format&gt;:[^}]+)?}", RegexOptions.Compiled); public StringTemplate(string template) { if (template == null) throw new ArgumentNullException("template"); this._template = template; } public static implicit operator StringTemplate(string s) { return new StringTemplate(s); } public override string ToString() { return _template; } public string Format(IDictionary&lt;string, object&gt; values) { if (values == null) { throw new ArgumentNullException("values"); } Dictionary&lt;string, int&gt; indexes = new Dictionary&lt;string, int&gt;(); object[] array = new object[values.Count]; int i = 0; foreach (string key in values.Keys) { array[i] = values[key]; indexes.Add(key, i++); } MatchEvaluator evaluator = (m) =&gt; { if (m.Success) { string key = m.Groups["key"].Value; string format = m.Groups["format"].Value; int index = -1; if (indexes.TryGetValue(key, out index)) { return string.Format("{{{0}{1}}}", index, format); } } return string.Format("{{{0}}}", m.Value); }; string templateWithIndexes = _regex.Replace(_template, evaluator); return string.Format(templateWithIndexes, array); } private static IDictionary&lt;string, object&gt; MakeDictionary(object obj) { Dictionary&lt;string, object&gt; dict = new Dictionary&lt;string, object&gt;(); foreach (var prop in obj.GetType().GetProperties()) { dict.Add(prop.Name, prop.GetValue(obj, null)); } return dict; } public string Format(object values) { return Format(MakeDictionary(values)); } public static string Format(string template, IDictionary&lt;string, object&gt; values) { return new StringTemplate(template).Format(values); } public static string Format(string template, object values) { return new StringTemplate(template).Format(values); } } </code></pre>
 

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