Regex nested backreference

Question

Is it possible to get a nested back reference to the below regex:

<field(.*)name="(.*)EndTime"(.*)\n((.*)\n)*?<property name="fieldClass" value="java.lang.String"/>\n</field>

Ie the "((.*)\n)*?"

Don't parse XML with RegEx! See stackoverflow.com/questions/8577060/… — AlexR
– AlexR, Commented Aug 11, 2016 at 16:11

Sebastian Lenartowicz · Accepted Answer · 2016-08-11 16:16:38Z

8

Yes, this is quite possible. Just be certain to watch for which numbered group you're using. Capturing groups (and thus backreferences) are numbered according to which opening bracket comes first - so, in this case, the outer brackets would yield \1, and the inner ones would yield \2.

For example, the pattern ((.+)ab)\1\2 will match the string 1234ab1234ab1234. The first capture group will match the 4 numbers plus the ab, while the second (inner) capture group will only match the numbers. Then we repeat each one, yielding the full match.

edited Aug 11, 2016 at 16:16

answered Aug 11, 2016 at 16:03

Sebastian Lenartowicz

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3 Comments

R Nar Over a year ago

Might also be worth mentioning named capturing groups to avoid getting mixed up on which group is which and being able to explicitly reference a certain group.

IanWatson Over a year ago

I don't quite understand what you mean here. Can you provide a example?

R Nar Over a year ago

you can name your capturing groups and back-reference them with those names. Although not directly answering the question put forward, I thought it was worth mentioning to avoid any confusion caused by the nested brackets. You can then do something like (?<first>ab(?<second>ba)) to have ${first} == abba and ${second} == ba

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