Java 9 Regular Expressions

The effect of eager matching on regular expression alternation

This regular expression engine behavior may return unexpected matches in alternation if alternations are not ordered carefully in the regex pattern.

Take an example of this regex pattern, which matches the strings white or whitewash:

white|whitewash

While applying this regex against an input of whitewash, the regex engine finds that the first alternative white matches the white substring of the input string whitewash, hence, the regex engine stops proceeding further and returns the match as white.

Note that our regex pattern has a better second alternative as whitewash, but due to the regex engine's eagerness to complete and return the match, the first alternative is returned ...