Avoid repeat for writing regex in java for match keywords for multiple pattern?

Question

I have N keywords, key1 key2 ... keyN.

I have 3 pattern to match the keywords. (To make the problem simple I just use key1 instead of real keyword)

/keyword {keyword} (keyword)

I decide to find the match pattern by simply using regex:

The pattern I wrote:

(?:/)(key1|key2|key3)|(?:{)(key1|key2|key3)(?:})|(?:\()(key1|key2|key3)(?:\))

But for the keyword(key1|key2|key3), I need to write 3 times, also I got 3 groups (should be reduce to 1 for the best result).

How can I achieve this in Java or the standard regular expression ?

Use string concatenation to build your regex. There is no better way in Java regex. — nhahtdh
– nhahtdh, Commented Jun 17, 2015 at 8:43
possible duplicate of Java Regex Multiple Pattern Group Replace — Rajesh
– Rajesh, Commented Jun 17, 2015 at 8:51
possible duplicate of stackoverflow.com/questions/9723277/… — Rajesh
– Rajesh, Commented Jun 17, 2015 at 8:53

Community · Accepted Answer · 2017-05-23 11:58:32Z

0

It is possible (if I didn't miss any edge cases), but might be a bit cumbersome.

You can try this regex:

"([{/(])(aaa|bbb|ccc)((?<=/(aaa|bbb|ccc))|(?<=\\((aaa|bbb|ccc))\\)|(?<=\\{(aaa|bbb|ccc))\\})"

([{/(]) matches /, (, {
(aaa|bbb|ccc) matches your key (group 2)
(?<=/(aaa|bbb|ccc)) matches nothing, but has a lookbehind that makes sure that / is at the beginning
- (?<=\\((aaa|bbb|ccc))\\) matches ) and with the lookbehind makes sure that ( is at the beginning
- (?<=\\{(aaa|bbb|ccc))\\} matches } and with the lookbehind makes sure that { is at the beginning

Using the information from this question: Backreferences in lookbehind you can try this regex without repitions:

"([{/(])(aaa|bbb|ccc)((?<=(?=/\\2).{1,4})|(?<=(?=\\(\\2).{1,4})\\)|(?<=(?=\\{\\2).{1,4})\\})"

where .{1,4} should be as long as needed to match your keys

edited May 23, 2017 at 11:58

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answered Jun 17, 2015 at 9:27

user140547

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2 Comments

Gohan Over a year ago

Your answer used the key1|key2|key3 more than 3 times. I'll check your group trick

user140547 Over a year ago

Well unfortunately it is not possible to use backreferences in lookbehinds (at least in Java regex), so I am afraid there is no way to avoid that repition. But the match is always in group 2, so at least there are no if statements or so neceassary to retrieve the result

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