$ cat ./parse.py
#!/usr/bin/python

from pyparsing import Regex, locatedExpr # type: ignore

regex_to_find \
    = locatedExpr(Regex(r'some text'))('line1')

def parse(text):
    r = locatedExpr(regex_to_find)('regex_to_find')
    return [match for match in r.scanString(text)]

def str_insert(string: str, substring: str, at: int) -> str:
    return string[:at] + substring + string[at:]

def process_matches(matches, text: str) -> str:
    print('Replacing…\n')
    for m in reversed(matches): # overwrite it starting from the bottom
        # prepend new text
        i_start = m[0]['regex_to_find'][f'line1']['locn_start']
        text = str_insert(text, 'foobar ', i_start)
    return text

text_to_process = \
"""	some text
"""

if __name__ == "__main__":
    matches = parse(text_to_process)
    print(f'Got matches: {matches}')
    if not matches:
        raise Exception("No matches")
    new_text = process_matches(matches, text_to_process)
    print(new_text)
$ ./parse.py
Got matches: [(([([8, ([8, 'some text', 17], {'locn_start': [8], 'value': ['some text'], 'locn_end': [17]}), 17], {'locn_start': [8], 'value': [([8, 'some text', 17], {'locn_start': [8], 'value': ['some text'], 'locn_end': [17]})], 'locn_end': [17]})], {'regex_to_find': [([8, ([8, 'some text', 17], {'locn_start': [8], 'value': ['some text'], 'locn_end': [17]}), 17], {'locn_start': [8], 'value': [([8, 'some text', 17], {'locn_start': [8], 'value': ['some text'], 'locn_end': [17]})], 'locn_end': [17]})]}), 8, 17)]
Replacing…

Traceback (most recent call last):
  File "/home/constantine/Projects/baum/bds/./parse.py", line 32, in <module>
    new_text = process_matches(matches, text_to_process)
  File "/home/constantine/Projects/baum/bds/./parse.py", line 19, in process_matches
    i_start = m[0]['regex_to_find'][f'line1']['locn_start']
  File "/usr/lib/python3.9/site-packages/pyparsing.py", line 598, in __getitem__
    return self.__tokdict[i][-1][0]
KeyError: 'line1'