string_trmplate_advanced.py

import string

class MyTemplate(string.Template):
    delimiter = '%'
    idpattern = '[a-z]+_[a-z]+'

template_text = '''
Delimiter   : %%
Replaced    : %with_underscore
Ignored     : %notunderscored
'''

d = {
    'with_underscore': 'replaced',
    'notunderscore': 'not replaced',
}

t = MyTemplate(template_text)
print('Modified ID pattern:')
print(t.safe_substitute(d))