592 improvement hdf5 dataset as regex

[jmorice91]

This PR correspond to fix the comment in the previous PR #583

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[Yushan-Wang]

Shall we add a reference to the regex's standard?

[jbigot]

Patterns and replacement strings support the following regular expression grammars:

• Modified ECMAScript regular expression grammar. This is the default grammar.
• Basic POSIX regular expression grammar.
• Extended POSIX regular expression grammar.
• The regular expression grammar used by the awk utility in POSIX.
• The regular expression grammar used by the grep utility in POSIX. This is effectively the same as the basic POSIX regular expression grammar, with the addition of newline '\n' as an alternation separator.
• The regular expression grammar used by the grep utility, with the -E option, in POSIX. This is effectively the same as the extended POSIX regular expression grammar, with the addition of newline '\n' as an alternation separator in addition to '|'.

Some grammar variations (such as case-insensitive matching) are also avaliable, see this page for details.

[jbigot]

If no grammar is chosen, ECMAScript is assumed to be selected

[jmorice91]

I propose to add at the end of this paragraph.

For the regex grammar, we used the default grammar in C++: ECMAScript.

What do you think?

[jbigot]

I'd put:

The regex use the Modified ECMAScript regular expression grammar.

[jmorice91]

Okay, with your proposition@jbigot.
I suggest to add explicitly the grammar when we create a regex:

std::regex dset_regex(dset_name_string, std::regex::ECMAScript);

instead of

std::regex dset_regex(dset_name_string);

Just in the case the default grammar changes in the future.