Comment 5 for bug 1171276

http://regexkit.sourceforge.net/Documentation/pcre/pcrepartial.html :

Consider, for example, an application where a human is required to type in data for a field with specific formatting requirements. An example might be a date in the form ddmmmyy, defined by this pattern:

  ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
If the application sees the user's keystrokes one by one, and can check that what has been typed so far is potentially valid,