/// <summary>
/// Replaces all occurrences of the regex in the string with the
/// replacement evaluator.
///
/// Note that the special case of no matches is handled on its own:
/// with no matches, the input string is returned unchanged.
/// The right-to-left case is split out because StringBuilder
/// doesn't handle right-to-left string building directly very well.
/// </summary>
private static string Replace(MatchEvaluator evaluator, Regex regex, string input, int count, int startat)
{
if (evaluator == null)
{
throw new ArgumentNullException(nameof(evaluator));
}
if (count < -1)
{
throw new ArgumentOutOfRangeException(nameof(count), SR.CountTooSmall);
}
if (startat < 0 || startat > input.Length)
{
throw new ArgumentOutOfRangeException(nameof(startat), SR.BeginIndexNotNegative);
}
if (count == 0)
{
return(input);
}
Match match = regex.Match(input, startat);
if (!match.Success)
{
return(input);
}
else
{
Span <char> charInitSpan = stackalloc char[ReplaceBufferSize];
var vsb = new ValueStringBuilder(charInitSpan);
if (!regex.RightToLeft)
{
int prevat = 0;
do
{
if (match.Index != prevat)
{
vsb.Append(input.AsSpan(prevat, match.Index - prevat));
}
prevat = match.Index + match.Length;
string result = evaluator(match);
if (!string.IsNullOrEmpty(result))
{
vsb.Append(result);
}
if (--count == 0)
{
break;
}
match = match.NextMatch();
} while (match.Success);
if (prevat < input.Length)
{
vsb.Append(input.AsSpan(prevat, input.Length - prevat));
}
}
else
{
// In right to left mode append all the inputs in reversed order to avoid an extra dynamic data structure
// and to be able to work with Spans. A final reverse of the transformed reversed input string generates
// the desired output. Similar to Tower of Hanoi.
int prevat = input.Length;
do
{
if (match.Index + match.Length != prevat)
{
vsb.AppendReversed(input.AsSpan(match.Index + match.Length, prevat - match.Index - match.Length));
}
prevat = match.Index;
vsb.AppendReversed(evaluator(match));
if (--count == 0)
{
break;
}
match = match.NextMatch();
} while (match.Success);
if (prevat > 0)
{
vsb.AppendReversed(input.AsSpan(0, prevat));
}
vsb.Reverse();
}
return(vsb.ToString());
}
}