Check if string is a prefix of a Javascript RegExp

Question

In Javascript I have defined a regular expression and now a user is typing in a string. I want to tell him if his string still could match the RegExp if he continues typing

Answer 1

I think your best bet here is to make your Regex prefix-proof. For the example you gave, /a*b/, I think you could probably use /a*b?/.test(userinput). For more complex patterns this could get increasingly difficult, but I still think it can be done by nesting each subexpression in a series of optional quantifiers (?). For instance:

/a*bcd*e/

The prefix regex could be:

/a*(b(c(d*e?)?)?)?/

Its a little messy, but will solve your problem rather well I think.