Vulnerability Instance

PyMdown Extensions has a ReDOS bug in its Figure Capture extension
The original issue came through PyMdown Extensions' normal issue tracker instead of the typical security flow: https://github.com/facelessuser/pymdown-extensions/issues/2716. Because this came through the normal issue flow, it was handled as a normal issue. In the future, PyMdown Extensions will ensure such issues, even if prematurely made public through the normal issue flow, are redirected through the typical security process.

The regular expression pattern in question is as follows:

```py
RE_FIG_NUM = re.compile(r'^(\^)?([1-9][0-9]*(?:.[1-9][0-9]*)*)(?= |$)')
```

The POC was provided by @ShangzhiXu

```py
import re
import time

regex_pattern = re.compile(r'^(\^)?([1-9][0-9]*(?:.[1-9][0-9]*)*)(?= |$)')

for i in range(50, 500, 50):
long_string = '1' * i + 'a'
start_time = time.time()
match = re.match(regex_pattern, long_string)
end_time = time.time()
print(f"long_string execution time: {end_time - start_time:.6f} s")
```

The issue with the above pattern is that `.` was used, which accepts any character when we meant to use `\.`. The fix was to update the pattern to:

```py
RE_FIG_NUM = re.compile(r'^(\^)?([1-9][0-9]*(?:\.[1-9][0-9]*)*)(?= |$)')
```

Relevant PR with fix: https://github.com/facelessuser/pymdown-extensions/pull/2717