[C++] CSV reader: support for multi-character / whitespace delimiter?

[asfimport]

I don't know how useful general "multi-character" delimiter support is, but one specific type of it that seems useful is "whitespace delimited", meaning any whitespace (possibly multiple / different whitespace characters).

In pandas you can achieve this either by passing delimiter="\s+" or specifying delim_whitespace=True (and both are equivalent, pandas special cases delimiter="\s+" as any other multi-character delimiter is interpreted as an actual regex and triggers the slower python engine intead of using the default c engine)

cc @pitrou @nealrichardson

Reporter: Joris Van den Bossche / @jorisvandenbossche

_{Note: This issue was originally created as ARROW-10432. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
Is that actually useful in the real world?

[asfimport]

Wes McKinney / @wesm:
Yes, it is. 

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Joris Van den Bossche / @jorisvandenbossche:
The "general multi-character" support I don't really know, but the specific case of "whitespace delimiter" certainly is. For example files that uses multiple spaces to have some alignment of columns in the plain text is not uncommon I think.