How to extract different variables from a regex without compiling each expression - regex

Question

I have a struct representing sizes of computer objects. Objects of this struct are constructed from string values input by users; e.g. "50KB" would be tokenised into an int value of "50" and the string value "KB".
type SizeUnit string
const (
B = "B"
KB = "KB"
MB = "MB"
GB = "GB"
TB = "TB"
)
type ObjectSize struct {
NumberOfUnits int
Unit SizeUnit
}
func NewObjectSizeFromString(input_str string) (*ObjectSize, error)
In the body of this function, I first check if the input value is in the valid format; i.e. any number of digits, followed by any one of "B", "KB", "MB", "GB" or "TB". I then extract the int and string components separately and return a pointer to a struct.
In order to do these three things though, I'm having to compile the regex three times.
The first time to check the format of the input string
rg, err := regexp.Compile(`^[0-9]+B$|KB$|MB$|GB$|TB$`)
And then compile again to fetch the int component:
rg, err := regexp.Compile(`^[0-9]+`)
rg.FindString(input_str)
And then compile again to fetch the string/units component:
rg, err := regexp.Compile(`B$|KB$|MB$|GB$|TB$`)
rg.FindString(input_str)
Is there any way to get the two components from the input string with a single regex compilation?
The full code can be found on the Go Playground.
I should point out that this is an academic question as I'm experimenting with Go's regex library. For a simple use-case of this sort, I would probably use a simple for loop to parse the input string.

Answer 1

You can capture both the values with a single expression using regexp.FindStringSubmatch:
func NewObjectSizeFromString(input_str string) (*ObjectSize, error) {
var defaultReturn *ObjectSize = nil
full_search_pattern := `^([0-9]+)([KMGT]?B)$`
rg, err := regexp.Compile(full_search_pattern)
if err != nil {
return defaultReturn, errors.New("Could not compile search expression")
}
matched := rg.FindStringSubmatch(input_str)
if matched == nil {
return defaultReturn, errors.New("Not in valid format")
}
i, err := strconv.ParseInt(matched[1], 10, 32)
return &ObjectSize{int(i), SizeUnit(matched[2])}, nil
}
See the playground.
The ^([0-9]+)([KMGT]?B)$ regex matches
^ - start of string
([0-9]+) - Group 1 (this value will be held in matched[1]): one or more digits
([KMGT]?B) - Group 2 (it will be in matched[2]): an optional K, M, G, T letter, and then a B letter
$ - end of string.
Note that matched[0] will hold the whole match.