import re

text = """"
\nword1     meanings
\nword123         measings
\nword12345        meanings
"""

maxLen = len(max(re.findall(r"^\S+", text, re.M), key=len))
result = re.sub(r"(\S+)[^\S\r\n]+", lambda m: '{{:{}s}}'
                .format(str(maxLen + 1))
                .format(m.group(1)),
                text)
print(result)