Escaping

A dozen characters are special in regex. To match them literally, escape them with a backslash.

The special characters

. ^ $ * + ? { } [ ] \ | ( )

To match a literal dot (not "any character"), write \.:

>>> re.search(r"3.14", "3x14").group()    # . is "any char" — matches!
'3x14'
>>> re.search(r"3\.14", "3x14") is None   # \. is a literal dot — no match
True
>>> re.search(r"3\.14", "pi is 3.14").group()
'3.14'

All verified. The #1 regex bug: writing www.example.com when you mean www\.example\.com. The unescaped version matches wwwXexampleYcom too — usually harmless, occasionally a real bug in validation.

re.escape: let Python do it

Matching user input literally? Don't hand-escape — automate it:

>>> user_input = "price: $5.00 (sale!)"
>>> pattern = re.escape(user_input)
>>> pattern
'price:\\ \\$5\\.00\\ \\(sale!\\)'
>>> bool(re.search(pattern, "the price: $5.00 (sale!) today"))
True

Verified. re.escape backslashes everything special. Whenever the pattern comes from data rather than your head, escape it.

Raw strings: why r"..." always

In a normal Python string, "\d" is... actually fine (\d isn't a recognized escape, so Python leaves it). But "\b" is a backspace character, not a word boundary! Compare:

>>> "\b" == chr(8)      # backspace — NOT what you want
True
>>> r"\b" == "\\b"      # two characters: backslash, b — correct
True

Verified. Always write regex as raw strings (r"\b\d+"). It's not about \d — it's about the day you need \b and get a backspace instead.

In the shell: quoting

grep -E "3\.14" file.txt     # double quotes: backslash survives
grep -E '3\.14' file.txt     # single quotes: safest — shell touches nothing

Single quotes are the shell's raw strings. With double quotes, the shell may eat your backslashes before grep sees them ("\d" → d). Single-quote your patterns in the shell, same instinct as raw strings in Python.

Cheat sheet

To match	Write	Example
A literal dot	\.	3\.14
A literal backslash	\\	C:\\path
Literal parens	\( \)	\(555\)
A literal +/*/?	\+ \* \?	1\+1
Any user-supplied text	re.escape(text)	—

Next: Flags