best approach for my pattern match - regex

Question

So, I've built a regex which follows this:
4!a2!a2!c[3!c]
which is translated to
4 alpha character followed by
2 alpha characters followed by
2 characters followed by
3 optional character
this is a standard format for SWIFT BIC code HSBCGB2LXXX
my regex to pull this out of string is:
(?<=:32[^:]:)(([a-zA-Z]{4}[a-zA-Z]{2})[0-9][a-zA-Z]{1}[X]{3})
Now this is targeting a specific tag (32) and works, however, I'm not sure if it's the cleanest, plus if there are any characters before H then it fails.
the string being matched against is:
:32B:HsBfGB4LXXXHELLO
the following returns HSBCGB4LXXX, but this:
:32B:2HsBfGB4LXXXHELLO
returns nothing.
EDIT
For clarity. I have a string which contains multiple lines all starting with :2xnumber:optional letter (eg, :58A:) i want to specify a line to start matching in and return a BIC from anywhere in the line.
EDIT
Some more example data to help:
:20:ABCDERF Z
:23B:CRED
:32A:140310AUD2120,
:33B:AUD2120,
:50K:/111222333
Mr Bank of Dad
Dads house
England
:52D:/DBEL02010987654321
address 1
address 2
:53B:/HSBCGB2LXXX
:57A://AU124040
AREFERENCE
:59:/44556677
A line which HSBCGB2LXXX contains a BIC
:70:Another line of data
:71A:Even more
Ok, so I need to pass in as a variable the tag 53 or 59 and return the BIC HSBCGB2LXXX only!

Answer 1

Your regex can be simplified, and corrected to allow a character before the H, to:
:32[^:]:.?([a-zA-Z]{6}\d[a-zA-Z]XXX)
The changes made were:
Lost the look behind - just make it part of the match
Inserting .? meaning "optional character"
([a-zA-Z]{4}[a-zA-Z]{2}) ==> [a-zA-Z]{6} (4+2=6)
[0-9] ==> \d (\d means "any digit")
[X]{3} ==> XXX (just easier to read and less characters)
Group 1 of the match contains your target

Answer 2

I'm not quite sure if I understand your question completely, as your regular expression does not completely match what you have described above it. For example, you mentioned 3 optional characters, but in the regexp you use 3 mandatory X-es.
However, the actual regular expression can be further cleaned:
instead of [a-zA-Z]{4}[a-zA-Z]{2}, you can simply use [a-zA-Z]{6}, and the grouping parentheses around this might be unnecessary;
the {1} can be left out without any change in the result;
the X does not need surrounding brackets.
All in all
(?<=:32[^:]:)([a-zA-Z]{6}[0-9][a-zA-Z]X{3})
is shorter and matches in the very same cases.
If you give a better description of the domain, probably further improvements are also possible.