python - REGEX - Differences between `^`, `$` and `A`, ``

Question

As I know, re proposes the following boundary matches.

• ^ matches at the beginning of a line.
• $ matches at the end of a line.
• A matches the beginning of the input.
• matches the end of the input.

Can you give me a concret example showing a real difference between between ^, $ and A, ?

Answer 1

The difference only becomes apparent when you use the re.M or re.MULTILINE multiline flag:

>>> re.search(r'^word', 'Line one
word on line two
', flags=re.M)
<_sre.SRE_Match object at 0x10124f578>
>>> re.search(r'Aword', 'Line one
word on line two
', flags=re.M) is None
True

where ^ matched at the start of a line (following a newline). $ matches at the end of a line:

>>> re.search(r'word$', 'Line one word
Line two
', flags=re.M)
<_sre.SRE_Match object at 0x10123e1d0>
>>> re.search(r'word', 'Line one word
Line two
', flags=re.M) is None
True

From the documentation:

re.M
re.MULTILINE

When specified, the pattern character '^' matches at the beginning of the string and at the beginning of each line (immediately following each newline); and the pattern character '$' matches at the end of the string and at the end of each line (immediately preceding each newline). By default, '^' matches only at the beginning of the string, and '$' only at the end of the string and immediately before the newline (if any) at the end of the string.

A always matches at the start of the string regardless, always at the end.