Replace comma-separated func(...) arguments safely in Python regex: stop greediness, keep the rest intact

Learn to replace multi-argument func(...) at string start with Python regex: avoid greedy .+, use negated classes, keep content intact with precise stops.

Replacing comma-separated arguments inside func(...) while keeping the rest of the string intact looks like a textbook regex task, until it quietly drops parts of your input. The trap is greediness: patterns like .+ happily run past closing parentheses and swallow more than intended, especially when multiple and func(...) segments follow. Below is a clear way to constrain matching and make the replacement deterministic for strings that begin with func(...).

Problem setup

The goal is to detect func(...) calls that contain two or more comma-separated items at the start of a string and replace everything inside the parentheses with a single value coming from another column, keeping the rest of the string as-is.

A straightforward loop with a greedy pattern may look like this:

import re

for row_idx, src_val in enumerate(frame['text_a']):
    frame.loc[row_idx, 'text_b'] = re.sub(r'^(func\().+,.+(\).+)', fr'\1{src_val}\2', frame.loc[row_idx, 'text_b'])

On short inputs this might seem to work, but as soon as the string contains more segments like and func(c,d) or trailing content such as and func2(z), the pattern can leap over the intended closing parenthesis and remove intermediate func(...) fragments. That mismatch explains why locally crafted tests sometimes pass while real data produces uneven results.

Why the greedy approach fails

The core issue is the use of .+ around commas and closing parentheses. The dot can match any character, including ), so in longer strings it can span up to a later ) and capture unintended content such as b,c) and func(d,e,f. Because regex engines are greedy by default, they try to consume as much as possible, which is exactly what causes the middle func(...) expressions to disappear after substitution.

Constraining what can be consumed inside func(...) is key. Instead of .+, a negated character class prevents matching commas and closing parentheses. This forces the engine to stay within the current argument list and stop correctly at the first closing parenthesis.

Solution

Build the replacement string for each iteration first, then apply a pattern that matches only the initial func(...) with at least one comma inside it. The following pattern, designed for Python’s re module, matches strings that start with func(...), with two or more comma-separated items, and stops exactly at the correct closing parenthesis.

Pattern: ^func\([^),]+(,[^,)]+)+\)

This works because it starts at the beginning of the string, matches literal func(, consumes any characters except , and ), then repeats a comma followed by more non-, non-) characters one or more times, and finally matches the closing ). The rest of the string remains untouched.

Here is a self-contained example using the same logic:

import re

data_map = {
    'col_text': ['func(a,b) and func(c,d)', 'func(a) and func(c)', 'func(b) and func(c,d)', 'func(a,b,c) and func(d,e,f)'],
    'col_repl': ['e', 'b', 'a', 'g']
}

output_col = []
regex = r'^func\([^),]+(,[^,)]+)+\)'

for idx, val in enumerate(data_map['col_repl']):
    repl_str = 'func(%s)' % val
    updated = re.sub(regex, repl_str, data_map['col_text'][idx])
    output_col.append(updated)

print(output_col)

This yields the expected results where only the first func(...) with multiple arguments at the start is rewritten, and subsequent segments such as and func(c,d) or and func2(z) are preserved.

Why it matters

When processing semi-structured text at scale, overbroad patterns introduce subtle data loss that is hard to detect after the fact. A single greedy dot near a parenthesis or comma can silently remove middle expressions, and downstream logic will operate on incomplete information. Using precise character classes in place of .+ prevents accidental cross-boundary matches and makes the transformation predictable.

Takeaways

Constrain what can be matched inside func(...) by excluding , and ) so the engine cannot run past the intended boundary. Ensure the string begins with func(...) when that is a requirement by anchoring with ^. Build the replacement string before calling re.sub to keep the substitution logic clean and consistent. With these pieces in place, replacing multi-argument func(...) calls while preserving the rest of the string becomes reliable, even in inputs that chain multiple conditions.