Question

I wrote a regex that validates an input string. It must have a minimum length of 8 chars (composed by alphanumeric and punctuation chars) and it must have at least one digit and one alphabetic char. So I’ve come up with the regex:

^(?=.*[0-9])(?=.*[a-zA-Z])[a-zA-Z0-9-,._;:]{8,}$

Now I have to rewrite this regex in a language that doesn’t support lookahead, how should I rewrite that regex?

Valid inputs are:

1foo,bar
foo,bar1
1fooobar
foooobar1
fooo11bar
1234x567
a1234567

Invalid inputs:

fooo,bar
1234-567
.1234567

Answer 1

There are two approaches. One is to compose a single expression which handles all possible alternatives:

^[a-zA-Z][0-9][a-zA-Z0-9-,._;:]{6,}$
  |
^[a-zA-Z][a-zA-Z0-9-,._;:][0-9][a-zA-Z0-9-,._;:]{5,}$
  |
^[a-zA-Z][a-zA-Z0-9-,._;:]{2}[0-9][a-zA-Z0-9-,._;:]{4,}$

etc. This is a combinatoric nightmare, but it would work.

A much simpler approach is to validate the same string twice using two expressions:

^[a-zA-Z0-9-,._;:]{8,}$          # check length and permitted characters

and

[a-zA-Z].*[0-9]|[0-9].*[a-zA-Z]  # check required characters

EDIT: @briandfoy correctly points out that it will be more efficient to search for each required character separately:

[a-zA-Z]                         # check for required alpha

and

[0-9]                            # check for required digit