`re` patterns with improperly positioned Line Anchors ($,^) don't error during compilation, yield false positives when matched against the empty string (e.g: `.match("")`)

[anabelle2001]

Bug report

Bug description:

Summary

The re library in Python does not raise an error when attempting to parse an unmatchable that incorrectly uses line anchors ($ and ^).

re.compile("$$")  # Expected: Compilation Error
re.compile("^^") Expected: Compilation Error 

Furthermore, any regex composed entirely of line anchors will always match the empty string:

print(bool(re.compile("$$").match("")))   # Expected: Compilation Error, Got: True
print(bool(re.compile("$$$$$$").match(""))) # Expected: Compilation Error, Got: True

Expected Behavior

Compilation of invalid patterns using $ and ^ (e.g., "$^", "$$", "$$$$$$", etc.) should raise an error during re.compile().
Patterns containing only $ and ^ should not match an empty string unless explicitly designed to do so (e.g., via .* or equivalent).

Actual Behavior

Patterns with improper usage of line anchors ($ and ^) compile without error.
All patterns consisting only of $ and ^ unexpectedly match an empty string, when the only patterns in that group that should match are /$/, /^/, and /^$/

Suggested Fix

Update the re library to strictly validate anchor usage during re.compile() and raise errors for invalid patterns.
Ensure that patterns like "$^", "$$", and "$$$$$$" behave consistently and intuitively, aligning with common regex behavior.

More Test Cases

print(bool(re.compile("$$").match("")))   # Expected: Compilation Error, Got: True
print(bool(re.compile("$$$$$$").match(""))) # Expected: Compilation Error, Got: True
print(bool(re.compile("$$$$$$.*").match(""))) # Expected: Compilation Error, Got: True
print(bool(re.compile("$$$$$$.*").match("a"))) # Expected: False, Got: False
print(bool(re.compile("$$$$^^$$").match("")))  # Expected: Compilation Error, Got: True

here's an example of how other invalid regexes are handled:

print(bool(re.compile("[[").match(""))) # Expected: Compilation Error, Got: FutureWarning and Exception

CPython versions tested on:

3.10

Operating systems tested on:

Windows

[picnixz]

The documentation for match() says (emphasis mine):

If zero or more characters at the beginning of string match this regular expression, return a corresponding Match

So it's as expected. In addition, the pattern is valid so even if the behaviour may not be as you expected, there shouldn't be any compilation error (it's not an error per se).

@serhiy-storchaka could you confirm my analysis?

[serhiy-storchaka]

I confirm.

What made you think this must be an error, @anabelle2001? Have you encountered this behavior in any other regexp implementations?

[anabelle2001]

huh. looks like i was wrong here. the formal definition for regexes in perl, javascript, and xml specify that ^ and $ are assertions that the cursor is at the start / end of a line.