Question

I have a regex that correctly captures a slash followed by a number in a string. The capturing group portion of the regex looks like this:

\(\d)+\\??

(some digits after a slash up to, but not including, a question mark) and there is more to the regex before and after this capturing group. Now I want to also include in my capturing group a optional specific prefix (call it “abc_”):

• The entire prefix (all four characters) must be there to be included in the captured group
• If no prefix is present then the digit portion of the capturing group is still captured
• if the prefix is partially there or some other prefix is there then the string does not match the regex.

Some examples:

abc_12345  is captured
12345      is captured
ab_12345   fails to match the regex
abc_       fails to match the regex
abcd_      fails to match the regex

How do I construct this?

Answer 1

If I understand you correctly, you want this:

((?:abc_)?\\d+)[?]?

The ?: operator transforms the group into a non-catching group. I don’t understand the part with the partial prefix. If you allow any content in front of the regex, you cannot deny a certain optional prefix. You need to have a clear separator in front of the pattern, like a white space in order to deny a prefix.