Regex to match the first half of a UK postcode

Question

I need a regex to match the first half of a uk postcode, and it's possible just one half or a full post code could get passed in.

There is great way to match full postcode here: UK Postcode Regex (Comprehensive)

However I need to just get the first 3 or 4 characters of a postcode, with the possibility of half a post code or a postcode without spaces getting passed in, so any of these:

SL4 4BZ
SL44BZ - this will pull out just 'SL4'
SL4
DY10
DY10 3BD
DY10 3BD

The regex needs to match:

• if it's 5 characters long (excluding spaces), remove last 3 characters and the first 2 remaining characters are the half-a-postcode
• if it's 6 characters long (excluding spaces), remove last 3 characters and the first 3 remaining characters are the half-a-postcode
• if it's 7 characters long (excluding spaces), remove last 3 characters and the first 4 remaining characters are the half-a-postcode
• if it's 2, 3 or 4 characters long, keep it

But I can't quite get my head around how to write something this complex.

Answer 1

Updated for 2/5 characters as well, total coverage = 2/5 or 3/6 or 4/7

This works for the input samples.
Here are 2 variations of the same thing.

Results in Capture group 1

 #  (?m)^([A-Z0-9]{2,4})(?:\s*[A-Z0-9]{3})?$

 (?m)
 ^ 
 (                             # (1 start)
      [A-Z0-9]{2,4} 
 )                             # (1 end)
 (?: \s* [A-Z0-9]{3} )?
 $ 

Results in Capture group 0

 # (?m)^[A-Z0-9]{2,4}(?=(?:\s*[A-Z0-9]{3})?$)

 (?m)
 ^ 
 [A-Z0-9]{2,4} 
 (?=
      (?: \s* [A-Z0-9]{3} )?
      $ 
 )

@Smickie - Update - Regarding your question about inline-modifiers in JS.
(I'm not a JS expert but here is a simple example)
I don't believe inline modifiers construct are available. However, they can be added
via the flags parameter (enum) or in the /../flags notation.

You can see this example work in JSfiddle here --> http://jsfiddle.net/cdcv8uug/

var teststr = "SL\nSL 4BZ\nSL4BZ\nSL4\nSL4 4BZ\nSL44BZ\nDY10\nDY10 3BD\nDY10 3BD";

var arr = teststr.match( /^[A-Z0-9]{2,4}(?=(?:\s*[A-Z0-9]{3})?$)/mg );

var res = "Using match() -> Capture group 0\nteststr.match(/^[A-Z0-9]{2,4}(?=(?:\s*[A-Z0-9]{3})?$)/mg)\n";
for (var index = 0; index < arr.length; index++)
{
   res += index + ":  " +  arr[index] + "\n";
}

var re = /^([A-Z0-9]{2,4})(?:\s*[A-Z0-9]{3})?$/mg;
res += "\nUsing exec() -> Capture group 1\nvar re = /^([A-Z0-9]{2,4})(?:\s*[A-Z0-9]{3})?$/mg;\nre.exec(teststr)\n";

index = 0;
while ((arr = re.exec(teststr)) != null)
{
    res += index + ":  " + arr[1] + "\n";
    index++;
}

alert( res );