Question

I need to build a JavaScript regular expression with the following constraints:

• The input string needs to be at least 6 characters long
• The input string needs to contain at least 1 alphabetical character
• The input string needs to contain at least 1 non-alphabetical character

I’m seriously lacking a lookback feature in JavaScript. The thing I came up with:

((([a-zA-Z][^a-zA-Z])|([^a-zA-Z][a-zA-Z]))....)|
(.(([a-zA-Z][^a-zA-Z])|([^a-zA-Z][a-zA-Z]))...)|
(..(([a-zA-Z][^a-zA-Z])|([^a-zA-Z][a-zA-Z]))..)|
(...(([a-zA-Z][^a-zA-Z])|([^a-zA-Z][a-zA-Z])).)|
(....(([a-zA-Z][^a-zA-Z])|([^a-zA-Z][a-zA-Z])))

This looks pretty long. Is there a better way?

How I came to this:

1. Regex for alphabetical character is [a-zA-Z]
2. Regex for non-alphabetical character is [^a-zA-Z]
3. So I need to look for a [a-zA-Z][^a-zA-Z] or [^a-zA-Z][a-zA-Z]
   so (([a-zA-Z][^a-zA-Z])|([^a-zA-Z][a-zA-Z])).
4. I need to check for n preceding characters and 6-n succeeding characters.

Answer 1

/^(?=.{6})(?=.*[a-zA-Z])(?=.*[^a-zA-Z])/

This means:

^ – start of the string
(?= ... ) – followed by (i.e. an independent submatch; it won’t move the current match position)
.{6} – six characters (“start of string followed by six characters” implements the “must be at least six characters long” rule)
.* – 0 or more of any character (except newline – may need to fix this?)
[a-zA-Z] – a letter (.*[a-zA-Z] therefore finds any string with a letter anywhere in it (technically it finds the last letter in it))
[^a-zA-Z] – a non-letter character

In summary: Starting from the beginning of the string, we try to match each of the following in turn:

• 6 characters (if we find those, the string must be 6 characters long (or more))
• an arbitrary string followed by a letter
• an arbitrary string followed by a non-letter