Cannot get bash to match whitespaces in a regular expression that is delimited by [[ ]] in an if satement

I am developing what should be a simple script to read a file line by line, assess the contents of each line, and process the line data depending on it line number. For some reason, I cannot get a regex that matches white spaces. [:space:], [[:space:]], [:blank:], \s, \ , , and " " have all failed.

My data is formatted as follows (fastq format):

    @SRR573708.2 2 length=100
    AAAACGTTAATATTTATTGAAATTGTT
    +SRR573708.2 2 length=100
    HHHHHHHHHHHHHHHHHHHHHHHHHHH

I would like to reformat it to:

    @SRR573708.2/2
    AAAACGTTAATATTTATTGAAATTGTT
    +SRR573708.2/2
    HHHHHHHHHHHHHHHHHHHHHHHHHHH

It is important, however, that I check each line to make sure it is formatted correctly before printing it to a new file. My last attempt at generating a reformatted file produced some really bizzare results at the end of the file. My code is:

    i=1
    while read LINE; do
        if (( $i > 4 )); then break; fi
        if (( $i % 4 == 1 )); then
            if [[ $data =~ ^@SRR[0-9]{6}[[:blank:]] ]]; then
                awk -v IFS=" " -v OFS="" -v ORS="" -v SUFFIX=$SUFFIX -v OUTPUT_FILE=$OUTPUT_FILE ' {print $1,SUFFIX,"\n"    } ' <<< $data
                i=$(( $i + 1 ))
            else
                echo -e "error at line ${i}"; echo "${data}"; exit 1; fi
        elif (( $i % 4 == 2 )); then echo -e "$LINE" 
            i=$(( $i + 1 ))

        elif (( $i % 4 == 3 )); then
            echo $data
            awk -v IFS=" " -v OFS="" -v ORS="" -v SUFFIX=$SUFFIX -v OUTPUT_FILE=$OUTPUT_FILE ' {print $1,SUFFIX,"\n"  } ' <<< $data
            i=$(( $i + 1 ))

elif (( $i % 4 == 0 )); then echo -e "$LINE" 
    i=$(( $i + 1 ))

else
    echo -e "number of liness is not divisible by 4. Program Terminated.\nProblem encountered at line ${i}."
    exit 1
fi

done < $INPUT_FILE

I get the error message:

    error at line 1
    @SRR573708.2 2 length=100

Any suggestions as to how to match a whitespace in a regex if-statement, preferable matching only space and tab characters and not newline characters.