java regex not matching [duplicate]

Question

I have a bunch of files from which I need to read parts of the filename. Therefore I thought a great use for regex and grouping :D the file could be named kb123.pdf or KB123.pdf so there is always kb or KB in the beginning and then 3 digits followed by .pdf I have the following code, but get "OH NO"s

Pattern pattern = Pattern.compile("^kb|KB(\\d{3})\\.pdf"); Matcher matcher = pattern.matcher("kb165.pdf"); if(matcher.matches()) { System.out.println("YAY"); }else { System.out.println("OH NO"); }

can anyone please explain, why the regex from the debugger in correct format ("\" got transformed in the debugger to the resulting "") as it shows to me:

^kb|KB(\d{3})\.pdf

works on regex101, but not in java? https://regex101.com/r/uRHkd0/1

Try "^(kb|KB)(\\d{3})\\.pdf" because regex concatenation (ab) binds stronger than regex alternation (a|b). — Good Night Nerd Pride
– Good Night Nerd Pride, Commented Apr 22 at 12:46
Your regex101 example works, but not as you expect. See this one, with another example input: regex101.com/r/ohsZo6/1 — Good Night Nerd Pride
– Good Night Nerd Pride, Commented Apr 22 at 12:51
by the way, no need to use ^ (or $) if using matches(), since it will only match against the whole input string anyway — user85421
– user85421, Commented Apr 22 at 12:52

lance-java · Accepted Answer · 2025-04-22 12:49:15Z

3

You need to put your OR logic in brackets, best to use a non capturing group ?:

^(?:kb|KB)\d{3}\.pdf

Another option is to ignore case

(?i)^kb\d{3}\.pdf

answered Apr 22 at 12:49

lance-java

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user85421 Apr 22 at 12:53

but be aware that ignoring case will also match kB or Kb (maybe even desired...)

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