c# regex get strings that have a specific sequence of letters and digits -

i've searched around here haven't found satisfying answer i've decided ask. need search collection of strings find expressions like:

ab 12345 ac 12345

i've tried regex

[a][bc][ ]\d{5}

but unfortunately in strings there also

#xxab 1234567890123 ggab 12345678901 ab 123456 sab 123456789

and above expression gets these valid. i've found , tried

[a][bc][ ]\d{5}$

but not match anything.

so question is, there expression find strings 5 numbers followed except more numbers?

from answers think not clear in exposition, strings in middle of larger string set possible string must match:

my description data xxxab 12345bcd

and also

my second description data ref. ab 12345

and also

my third description data refab 12345

and also

ab 12345with description following ab 12345 space , description following

the strings uppercase in database.

use ?! lookahead.

a[bc] \d{5}(?!\d)

see demo.

M16