I am looking for a regular expression in python which will identify all the function bodies in a C file

I am looking for a RE in python which will identify all the C function

I want to automate the insertion of some comments at the start of each function,
For example a function looks like

static my_struct1* alloc_mem (my_struct2* a)
{

...

}

I want to insert comment and make it look like

static my_struct1* alloc_mem (my_struct2* a)
{
/* my comment */
...

}

So I want to identify all the head (ending with {) of function bodies and insert comment there.

I tried below code:

def insert_comment():
    comment = "/* my comment */"
    pattern = r'[^(if|else|switch|for|if\s+|else\s+|switch\s+|for\s+)]\(.*\)(\s|\n)*\{'
    matches = list(re.finditer(pattern1, content))
    for match in matches:
        print('*****')
        print(match.group())

with open(filename, "r") as i:
        content = i.read()
        insert_comment()

But this is also matching if and else statements which have nested ( expression.

For example,

if(MACRO(expa) && MACRO(expb)) {

Then the matched pattern will be

O(expb)) {

What might be a better RE to get start of function body?

As commented by others, C syntax is too complex to analyze without
a dedicated parser in a strict sense.
As for your requirements, however, it looks some under/over detection
may be acceptable (harmless), because it will not cause defects to
the original source code just to insert comments.

Here is a simple code to start with to insert comments at your desired points
under limited conditions:

#!/usr/bin/python

import regex

with open(filename) as f:
    s = f.read()
    m = regex.sub(r'\b(?:if|switch|for|while|until)\b(*SKIP)(*FAIL)|(([A-Za-z_]\w*\s*\((?:[^()]+|(?2))*\))\s*{)', r'\n/* my comment */', s)
    print(m)

[Explanations]

• regex is a PyPi module with additional regex functionalities.
• (?:if|switch|for|while|until) is the reserved names which has
  a syntax similar to C functions (to be excluded).
• \b(?:if|switch|for|while|until)\b(*SKIP)(*FAIL) discards these matches.
• (([A-Za-z_]\w*\s*\((?:[^()]+|(?2))*\))\s*{) is a recursive regex which
  matches C function names with balanced parentheses.

[Edit]
If you are sure the macro names are all uppercase letters, you can exclude them by tweaking the regex as:

#!/usr/bin/python

import regex

with open(filename) as f:
    s = f.read()
    m = regex.sub(r'\b(?:if|switch|for|while|until|[A-Z_]+)\b(*SKIP)(*FAIL)|(([A-Za-z_]\w*\s*\((?:[^()]+|(?2))*\))\s*{)', r'\n/* my comment */', s)
    print(m)

Input example:

static my_struct1* alloc_mem (my_struct2* a)
{ 
// function
}

some_function(foo) {
// function
}

if(MACRO(expa) && MACRO(expb)) {
// reserved word
}

TAILQ_FOREACH(entry, hent, next) {
// macro
}

while(true) {
// reserved word
}

Output:

static my_struct1* alloc_mem (my_struct2* a)
{
/* my comment */
// function
}

some_function(foo) {
/* my comment */
// function
}

if(MACRO(expa) && MACRO(expb)) {
// reserved word
}

TAILQ_FOREACH(entry, hent, next) {
// macro
}

while(true) {
// reserved word
}

[Explanation]

The regex [A-Z_]+, which matches the all uppercase letters, is appended to
the list of exclusion as:

(?:if|switch|for|while|until|[A-Z_]+)