Apex Regex Extract Numeric Value from String

You are using a capturing group so this works as you expect: (You need to use find):

Pattern pat = Pattern.compile('([0-9]+)');
    Matcher matcher = pat.matcher('red fox 11133434 red fox');
    Boolean matches = matcher.find();
    system.debug(logginglevel.error,matches);
    system.debug(logginglevel.error,matcher.group(1));

You will notice the debugs are true and 11133434

A good site to test out your regex and break it down it:

https://regex101.com

B"H

Thanks Eric for answering the question. Just for completeness I am sharing the definition difference between find and matches from the Java docs:

A matcher is created from a pattern by invoking the pattern's matcher method. Once created, a matcher can be used to perform three different kinds of match operations:

• The matches method attempts to match the entire input sequence against the pattern.
• The lookingAt method attempts to match the input sequence, starting at the beginning, against the pattern.
• The find method scans the input sequence looking for the next subsequence that matches the pattern.