Discord large text generator
Python 2, 275 268 267 bytes

d=['']*256
d[48:58]='zero one two three four five six seven eight nine'.split()
d[9]=d[10]=d[32]='white_large_square'
d[65:90]=d[97:122]=map(lambda c:'regional_indicator_'+chr(c),range(97,122))
print' '.join((':%s:'%d[ord(c)])*(d[ord(c)]!='')for c in input()).strip()


It creates a large list of strings, iterates through the input string's characters and gets the string from the list using the character's ord value.
Works for ascii characters only. Replaces space, tab and newline only; any other whitespace will be discarded.

It did leave whitespaces in front so I had to strip it in the end.

I also tried doing it with regex, turned out to be longer (286 bytes):

import re
print re.sub('([ \t\n])|([a-z])|(\d)',lambda m:':'+['white_large_square','regional_indicator_'+m.group(m.lastindex),'zero one two three four five six seven eight nine'.split()[(ord(m.group(m.lastindex))-68)%10]][m.lastindex-1]+': ',re.sub('[^a-z0-9 \t\n]','',input().lower()))