10-60X slower compared to java regexes in some cases

Original report: https://groups.google.com/forum/#!topic/re2j-discuss/8c3L06m6wbY

pattern is : "._d|e|cart|jinjian|kk._"
String is: "aajinjianaksdjflaajinjianaksdjflaajinjianaksdjfl"

Java takes: 330ms
re2j takes: 4257ms

Thanks for your time to take a look at it.

This came up also on https://github.com/prometheus/jmx_exporter/issues/23 @bbaja42