PEMDAS Recursive Algorithm Program - Negative Numbers Help

Hi, we have an assignment in my programming class that were are supposed to create a recursive algorithm that correctly processes the order of operations in the PEMDAS sequence. I have a program that runs and almost works properly except it does not correctly process negative numbers in the input or output. I am kind of confused as where to start for this without ruining the code I have so far. This is what I have written:

function PEMDAS = algebra(exp)
lefthead = 0
lefttail = 0
righthead = 0
righttail = 0
check = true
while check 
    for i = 1:length(exp)
        if i>length(exp)
            break
        end
       if exp(i) == '^'
            lefttail = i-1
            righthead = i+1
            for j = lefttail :-1:1
                if j == 1 
                    lefthead = j
                elseif exp(j) == '^' || exp(j) == '*' || exp(j) == '/' || exp(j) == '+' || exp(j) == '-'
                    lefthead = j+1
                    break
                end
            end
            for j = righthead :1:length(exp)
                if j == length(exp)
                    righttail = j
                elseif exp(j) == '^' || exp(j) == '*' || exp(j) == '/' || exp(j) == '+' || exp(j) == '-'
                    righttail = j-1
                    break
                end
            end
            subexp = exp(lefthead:righttail)
            val = evaluate(subexp)
            exp = [exp(1:lefthead-1), val, exp(righttail+1:end)]
        end
    end
    
    for i = 1:length(exp)
        if i>length(exp)
            break
        end
       if exp(i) == '*' || exp(i) == '/'
            lefttail = i-1
            righthead = i+1
            for j = lefttail :-1:1
                if j == 1 
                    lefthead = j
                elseif exp(j) == '^' || exp(j) == '*' || exp(j) == '/' || exp(j) == '+' || exp(j) == '-'
                    lefthead = j+1
                    break
                end
            end
            for j = righthead :1:length(exp)
                if j == length(exp)
                    righttail = j
                elseif exp(j) == '^' || exp(j) == '*' || exp(j) == '/' || exp(j) == '+' || exp(j) == '-'
                    righttail = j-1
                    break
                end
            end
            subexp = exp(lefthead:righttail)
            val = evaluate(subexp)
            exp = [exp(1:lefthead-1), val, exp(righttail+1:end)]
        end
    end
    
    for i =1:length(exp)
        if i>length(exp)
            break
        end
        if exp(i) == '+' || exp(i) == '-'
            lefttail = i-1
            righthead = i+1
            for j = lefttail :-1:1
                if j == 1 
                    lefthead = j
                elseif exp(j) == '^' || exp(j) == '*' || exp(j) == '/' || exp(j) == '+' || exp(j) == '-'
                    lefthead = j+1
                    break
                end
            end
            for j = righthead :1:length(exp)
                if j == length(exp)
                    righttail = j
                elseif exp(j) == '^' || exp(j) == '*' || exp(j) == '/' || exp(j) == '+' || exp(j) == '-'
                    righttail = j-1
                    break
                end
            end
            subexp = exp(lefthead:righttail)
            val = evaluate(subexp)
            exp = [exp(1:lefthead-1), val, exp(righttail+1:end)]
        end
    end
    
    for i = 1:length(exp)
        if exp(i) == '^' || exp(i) == '*' || exp(i) == '/' || exp(i) == '+' || exp(i) == '-'
            break
        elseif i==length(exp)
            check = false
        end
    end
end
PEMDAS = exp
end 

function pemdas = evaluate(subexp)
for i = 1:length(subexp)
    if subexp(i) == '^' || subexp(i) == '*' || subexp(i) == '/' || subexp(i) == '+' || subexp(i) == '-'
       opr = subexp(i)
       subexp(i) = ' '
        break
    end
end

nums = split(subexp)
num1 = str2num(nums{1})
num2 = str2num(nums{2})
switch opr
    case '^'
        do = num1^num2
    case '*'
        do = num1*num2
    case '/'
        do = num1/num2
    case '+'
        do = num1+num2
    case '-'
        do = num1-num2
end  
           
pemdas = num2str(do)
end 

Any help would be greatly appreciated!