text = "hello23 the2e are 13 5.12apples *specially_x00123 named"

text_cleaned = "hello## the#e are ## #.##apples *specially_x00123 named"

p1 = r'd(?<!*w+_w+)'

p2 = r'(?:*[a-z]+_w+)b|d'

# coding=utf8
# the above tag defines encoding for this document and is for Python 2.x compatibility

import re

regex = r"(d)"

test_str = "hello23 the2e are 13 5.12apples *specially_x00123 named"

subst = "#"

# You can manually specify the number of replacements by changing the 4th argument
result = re.sub(regex, subst, test_str, 0, re.MULTILINE)

if result:
    print (result)

# Note: for Python 2.7 compatibility, use ur"" to prefix the regex and u"" to prefix the test string and substitution.