how to make key file pair - collect files from different lanes using key?

[complexgenome]

For normal and tumor, I've forward and reverse read per sample from different sequencing lane. I'd like to merge different lanes of one sample into one. When I merge I would like to merge them in sorted manner, that is L003 before L004.

I've files as:

MM-2658-DNA-N-01-01_L003_R1_001.fastq.gz
MM-2658-DNA-N-01-01_L003_R2_001.fastq.gz
MM-2658-DNA-N-01-01_L004_R1_001.fastq.gz
MM-2658-DNA-N-01-01_L004_R2_001.fastq.gz

MM-4653-DNA-N-01-01_L003_R1_001.fastq.gz
MM-4653-DNA-N-01-01_L003_R2_001.fastq.gz
MM-4653-DNA-N-01-01_L004_R1_001.fastq.gz
MM-4653-DNA-N-01-01_L004_R2_001.fastq.gz

I've following code:

normal_reads=Channel.fromFilePairs("/sc/arion/projects/pareks02a/sanjeev/test_input_data/t_n/MM-*-N-*_L00*_R{1,2}_*.fastq.gz")

normal_keys = normal_reads.map { file ->
        def matcher = (file =~ /(MM-\d+)-DNA-N-.+/)
        matcher ? "${matcher[0][1]}" : ''
        }
normal_keys.view()

def collectedFiles = []

normal_reads.subscribe { file ->
        def matcher = (file =~ /(MM-\d+)-DNA-N-.+/)
        if (matcher) {
            def key = matcher[0][1]
            if (!collectedFiles.contains(key)) {
                collectedFiles[key] = []
            }
            collectedFiles[key] << file
        }
    }

I get error:

argument type mismatch

I'd eventually like to do:

zcat MM-2658-DNA-N-01-01_L003_R1_001.fastq.gz MM-2658-DNA-N-01-01_L004_R1_001.fastq.gz > MM-2658_R1.fastq
zcat MM-2658-DNA-N-01-01_L003_R2_001.fastq.gz MM-2658-DNA-N-01-01_L004_R2_001.fastq.gz > MM-2658_R2.fastq