How to find all words in text file which consist only of digits and replace them with a specific word

How to find all words (separated with spaces from both sides) in text file which consist only of digits and replace them with a specific word. I do like this if I want to replace a word to a word, but how about digit words? They can consist of different lengths. Thanks.

fin = open("tekstas.txt", "rt")

with open('tekstas.txt') as f:
    if ' ir ' in f.read():
        fout = open("naujasTekstas.txt", "wt")
        for line in fin:
            fout.write(line.replace(' '', ' skaičius '))
            fin.close()
            fout.close()
            f.close()
    else:
        print("Nėra žodžio "ir".")

Answer

Use regex, most easily available with Python’s re module:

import re

text = "word another word 121434"

pattern = '[0-9]+'

print(re.sub(pattern=pattern, repl='replacement', string=text)) # word another word replacement

Link to step by step analysis of this regex: https://regex101.com/r/d7rljs/1