scala.MatchError when partitioning a string with an optional part into one or three parts

I am trying to pull out something like this:

params = {"path", "contentName"}

part of parametersStr below

@RequestMapping(value = "/breezeQuery", params = {"path", "contentName"}, method = RequestMethod.GET)

Why is this code giving me a scala.MatchError?:

val paramsPattern = """(.*)(?:params = \{.*})?(.*)""".r
val paramsPattern(left, paramsStr, right) = parametersStr

Also, the pattern like this may not occur in the string. So I also want to know if that is the case. Finally, I'm capturing everything to the left and right of the group so that I can concatenate them to remove the captured group from the string. It is optional, but I do want to capture it if it is present.

Solution

I believe you want to partition the string into 3 or 2 parts (depending on the optional params = \{.*}).

You may use

^(.*?)(?:(params\s*=\s*\{.*?})(.*))?$

See the regex demo. Details

^ - start of string
(.*?) - Group 1 (left):
(?:(params\s*=\s*\{.*?})(.*))? - an optional non-capturing group, will be tried at least once:
- (params\s*=\s*\{.*?}) - Group 2 (paramsStr):params word, = enclosed with 0+ whitespaces, {, any zero or more chars other than line break chars, as fewas possible and then }
- (.*) - Group 3: any zero or more chars other than line break chars, as many as possible
$ - end of string

See the Scala demo:

val parametersStr = """@RequestMapping(value = "/breezeQuery", params = {"path", "contentName"}, method = RequestMethod.GET)"""
val paramsPattern = """^(.*?)(?:(params\s*=\s*\{.*?})(.*))?$""".r
val paramsPattern(left, paramsStr, right) = parametersStr
println(s"Left: $left\nParam String: $paramsStr\nRight: $right")

Output:

Left: @RequestMapping(value = "/breezeQuery", 
Param String: params = {"path", "contentName"}
Right: , method = RequestMethod.GET)