Different behavior for '^'

[sk-]

In plain ruby we have:

Regexp.new('^a').match("\na")
 => #<MatchData "a">

whereas in re2:

RE2::Regexp.new('^a').match("\na")
 => false

[mudge]

This is due to a difference in the underlying Google re2 library's syntax which isn't 100% compatible with PCRE: in order for ^ to match the beginning of a line, you need to use the multi-line mode flag.

RE2::Regexp.new('(?m)^a').match("\na")
=> true

Or, using a capturing group to return the match:

RE2::Regexp.new('(?m)(^a)').match("\na")
=> #<RE2::MatchData "a" 1:"a">

[sk-]

Could you add a section to the Readme stating the differences and incompatibilities of re2 compared to ruby's Regexp.