The following issue is observed only on Java and not on other regex flavors (e.g PCRE).
I have the following regex: (?:(?<=([A-Za-z\d]))|\b)(MyString)
. There's a capturing group on [A-Za-z\d]
in the lookbehind.
And I'm trying to match (through Pattern.matcher(regex)
; to be precise, I'm calling replaceAll
) the following string: string.MyString
.
On PCRE, I will match MyString
, and it will be the second group in the match. On Java, however, I will match the g
in string
as group 1, and MyString
as group 2.
[A-Za-z\d]
should only be matched if it directly precedes MyString
, which is not the case here.g
? I want to keep the capturing group in case I have to match a string like stringMyString
, in which case I do need that g
as group 1.There is a line on the java.util.regex.Pattern docs
The captured input associated with a group is always the subsequence that the group most recently matched. If a group is evaluated a second time because of quantification then its previously-captured value, if any, will be retained if the second evaluation fails. Matching the string "aba" against the expression (a(b)?)+, for example, leaves group two set to "b". All captured input is discarded at the beginning of each match.
I think this line explains the behavior:
The last line:
So if you have this string:
string.MyString.srting.MyString
And this regex:
(?:(?<=([tr]))|\b)(MyString)
You can see that the group 1 value is different in both matches as all captured input is discarded.
See an example on regex101