Input Validation

[znd4]

Is your feature request related to a problem? Please describe.
If step 5 in my script requires a particular type of free-text input, (e.g. URL, S3 bucket, semver) if the user enters an invalid input, they'll have to start the form over (unless I implement a custom REPL for that input, or save WIP forms to disk)

Describe the solution you'd like

I'd like to be able to provide a regular expression to gum input, e.g.

# semver pattern, see https://semver.org/#is-there-a-suggested-regular-expression-regex-to-check-a-semver-string
PATTERN='^(?P<major>0|[1-9]\d*)\.(?P<minor>0|[1-9]\d*)\.(?P<patch>0|[1-9]\d*)(?:-(?P<prerelease>(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*)(?:\.(?:0|[1-9]\d*|\d*[a-zA-Z-][0-9a-zA-Z-]*))*))?(?:\+(?P<buildmetadata>[0-9a-zA-Z-]+(?:\.[0-9a-zA-Z-]+)*))?$'
gum input --regex $PATTERN

If the user enters an invalid input, there should be some kind of (accessible) feedback -- like flashing a warning symbol. Additionally, this might require a customizable warning message, e.g.:

gum input ... --regex-message "Invalid input. Please follow semver (e.g.: 1.2.3)"

Describe alternatives you've considered

Save intermediate inputs to disk and prompt users at start

# my_script.py
import subprocess as sp
import datetime as dt

form_body = {}
if previous_form_on_disk() and user_wants_to_use_saved_form():
    form_body() = select_form()

current_save_path = local_data_path() / f"{dt.datetime.now()}"
    
form_body["field1"] = form_body.get("field1", prompt_1())
save(current_save_path, form_body)
# ...

implement REPL for the tricky gum inputs

def prompt_n():
    header = ""
    while True:
        result = sp.check_output(["gum", "input", ...])
        if re.match(result, r"..."):
            return result
        header="invalid input. Please follow ..."

Additional context

This would definitely require some design work, and I'm not sure if there's a generic enough solution to warrant including this as a feature (instead of coming up with a nifty documentation snippet)

[znd4]

This might also require upstream work in huh, depending on the design