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using System;
using System.Linq;
using System.Text.RegularExpressions;
using System.Text;

string str = @"
a1 = b1;
a2 = b2;
a3 = b3;";

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    /// <summary>Creates a valid Windows filename from a string.</summary>
    /// <param name="filename">The filename candidate.</param>
    /// <param name="replaceInvalidCharsWith">What to replace invalid filename chars in <paramref name="filename"/> with.</param>
    /// <returns>
    /// A new <see cref="string"/>, equivalent to <paramref name="filename"/>, but modified to be a valid Windows filename if it
    /// <paramref name="filename"/> wasn't already.
    /// </returns>
    /// <exception cref="ArgumentException"/>
    /// <remarks>The length of the filename is not checked, and the casing is not modified.</remarks>
    public static string ToFilename(this string filename, string replaceInvalidCharsWith = "_")