regexp/syntax: add examples for flag parsing

[kevinburke]

Reading the description of grouping and flags, I'm confused about how those flags would apply to theoretical strings. An example or two would be good to demonstrate the use and placement of flags, as well as the resulting capture groups.

[ysmolski]

@kevinburke. I would like to tackle it, but have a couple of questions.

Maybe you meant the regexp package? It also lacks examples for Perl/POSIX syntax and groupings. I am asking because for me it's hard to see how to show a concise example based on regexp/syntax only. The output of regexp/syntax functions are structures indented for machine consumption.