RegExp has no convenient way to see if a substring starts with a match to RegExp

[rakudrama]

RegExp re = ...

I want to say something like
Match m = re.matchAt(str, pos);
or
String s = re.matchStringAt(str, pos);

The match should be tethered to the given position, i.e. succeed only if str[pos ... end) matches the re.

Currently all the RegExp methods do a search and none do a simple match. It is inconvenient to have to specify regexps with a leading ^ and there is no convenient way to convert a RegExp without a leading ^ into one with.

None of search methods allow a search in a substring. This would be OK if string.substring was guaranteed constant time + space, but the documentation on String.substring makes no claims.

[rakudrama]

Removed the owner.
Removed Type-Defect label.
Added Type-Enhancement, New labels.

[DartBot]

This comment was originally written by drfibonacci@google.com

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[lrhn]

I've added a method to Pattern called matchAsPrefix which also takes an optional start position. That means you can do:
  Match m = re.matchAsPrefix(str, pos);
and get what you want.

Fair warning: It's still not efficient on the VM where it just does a search and checks if the start of a match is pos. I hope we'll eventually get a RegExp implementation that's more efficient, or maybe use the same hack that is used for JS RegExps.

You can create a modified RegExp by using the pattern and flags of the original.
  var re = ...some regexp ...
  var anchorRe = new RegExp("^(?:${re.pattern})", multiline: re.iSMultiline,
                                                  caseSenitive: re.isCaseSensitive);

Prefixing with "^" wouldn't work though - it only matches the start of the string, which is never at a position > 0. What we do for JS RegExp is to suffix with "|()", so there is a guaranteed zero-length match at the start position if the original regexp didn't match. This prevents searching, and allows us to recognize that the original regexp didn't match if the extra capture participated.

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