Java Regex to Parse a Path Into Multiple Optional Groups

Question

I am trying to split this type string using a Java Regex:

/api/vX/client/domain/category/id

crudely into this:

(?:/api)?(?:/vX)?(/client/domain/...)?(?:/category)?(?:...)?

I would split it into the following groups:

[0] /api
[1] /vX (1-x)
[2] /client/domain (/a/b/...) (up to category below)
[3] /category1 | /category2
[4] /everything else

Right now, I am trying to us a regex like this but it is just not working the way I am expecting it to.

(\/api)?(\/v\d+)?(\/\w+)(\/category1|category2\/?.*)?

I also need to take into account trailing/leading slashes with the expectation a leading slash will always start a segment but a trailing slash may or may not be there (unless there is a next segment).

Some example of paths and outputs I am trying to achieve are:

/client: 
[0], [1], [2]=/client, [3], [4]

/api/client: 
[0]=/api, [1], [2]=/client, [3], [4]

/api/v1/client/domain: 
[0]=/api, [1]=/v1, [2]=/client, [3], [4]

/api/v1/client/domain/category1: 
[0]=/api, [1]=/v1, [2]=/client/domain, [3]=/category1, [4]

api/v1/client/d1/d2/d3/category1: 
[0]=/api, [1]=/v1, [2]=/client/d1/d2/d3, [3]=/category1, [4]

/api/v2/client/domain/category2/id: 
[0]=/api, [1]=/v2, [2]=/client/domain, [3]=/category2, [4]=/id

Have a look at Regexplanet, idea maybe helpful. Click on Java. — bobble bubble
– bobble bubble, Commented Nov 15, 2015 at 5:08

Mariano · Accepted Answer · 2015-11-15 05:36:58Z

The following regex will match what you defined:

 ^(/api)?(/v\d+)?(/[^/]+(?:/[^/]+)*?)??(?:(/category[12])(/.*)?)?$

^ matches the start of line
(/api)? group 1 (optional)
(/v\d+)? group 2 (optional)
(/[^/]+(?:/[^/]+)*?)?? group 3 matches any number of groups, for client, domain, etc. (optional)
- Both the outter and the inner group have a lazy quantifier, to allow a match in categories.
- [^/]+ is a character class that matches anything except slashes.
(?:(/category[12])(/.*)?)? is an optional non capturing group that matches:
- (/category[12]) category1 or 2 in group 4
- (/.*)? group 5: anything else (optional)
$ the end of string (this is important to force lazy matches to capture all the text)

Code

String text = "/api/v2/client/domain/category2/id";
String pattern = "^(/api)?(/v\\d+)?(/[^/]+(?:/[^/]+)*?)??(?:(/category[12])(/.*)?)?$";
Pattern regex = Pattern.compile(pattern, Pattern.CASE_INSENSITIVE);
Matcher m = regex.matcher(text);

while (m.find())
{
    System.out.println("api: " + m.group(1) + 
                       "\nversion: " + m.group(2) +
                       "\nclient: " + m.group(3) +
                       "\ncategory: " + m.group(4) +
                       "\nextra: " + m.group(5));
}

Output

api: /api
version: /v2
client: /client/domain
category: /category2
extra: /id

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