Assuming you don’t need very strict input validation, since you are converting from the format with th on the number (or st or nd in 31st, 2nd and more), I suggest you simply remove those two letters first. A regex may do that:
// remove st, nd, rd or th after day of month
dateInString
= dateInString.replaceFirst("^(\\d+)(st|nd|rd|th)( \\w+ \\d+)$", "$1$3");
String dateOutString = LocalDate.parse(dateInString,
DateTimeFormatter.ofPattern("d MMM uuuu", Locale.ENGLISH))
.format(DateTimeFormatter.ofPattern("dd/MM/uuuu"));
The result is
26/05/2017
This works if your input contains a three letter abbreviation for the month, like Apr, May or Jun. To accept a full month name instead (April, May, June), you need 4 Ms instead of 3 in the format pattern: d MMMM uuuu.
