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matchAsPrefix abstract method
Match matchAsPrefix(Match this pattern against the start of string.
If start is provided, it must be an integer in the range 0 .. string.length. In that case, this patten is tested against the string at the start position. That is, a Match is returned if the pattern can match a part of the string starting from position start. Returns null if the pattern doesn't match.
Source
Match matchAsPrefix(String string, [int start = 0]);
  © 2012 the Dart project authors
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 https://api.dartlang.org/stable/1.24.3/dart-core/Pattern/matchAsPrefix.html