Flags

Flags change how the engine interprets your pattern. Three cover nearly everything.

re.IGNORECASE: case doesn't matter

>>> re.findall(r"cat", "Cat CAT cat")
['cat']
>>> re.findall(r"cat", "Cat CAT cat", re.IGNORECASE)
['Cat', 'CAT', 'cat']

Verified. (re.I is the short form.) For user input and log scanning, this is almost always what you want.

re.MULTILINE: ^ and $ per line

Without it, ^ matches only at the start of the string. With re.MULTILINE, it matches at the start of every line:

text = "error: disk full\nwarning: low mem\nerror: timeout"
>>> re.findall(r"^error.*", text)
['error: disk full']
>>> re.findall(r"^error.*", text, re.MULTILINE)
['error: disk full', 'error: timeout']

Verified. (re.M.) Without MULTILINE, ^ matches only at the start of the string — you get just the first line. With it, ^ matches at the start of every line. The classic use: "every line starting with ERROR" in a multi-line log.

re.DOTALL: dot matches newlines too

. matches everything except \n by default. re.DOTALL removes the exception:

>>> re.search(r"start.*end", "start\nmiddle\nend") is None
True
>>> re.search(r"start.*end", "start\nmiddle\nend", re.DOTALL).group()
'start\nmiddle\nend'

Verified. (re.S.) Needed when your match spans lines — multi-line quoted strings, HTML blocks (though see the parser warning in chapter 1).

Combining and inline flags

>>> re.findall(r"^error.*", text, re.IGNORECASE | re.MULTILINE)
['error: disk full', 'error: timeout']

Verified — flags combine with |. You can also write them inline at the pattern's start: (?i) = IGNORECASE, (?m) = MULTILINE, (?s) = DOTALL:

>>> re.findall(r"(?im)^error.*", text)
['error: disk full', 'error: timeout']

Verified. Inline flags are handy in tools (grep, editors) where there's no flags argument — grep -i and grep -E cover the common cases there.

The flag cheat sheet

Flag	Short	Effect
re.IGNORECASE	re.I / (?i)	Case-insensitive
re.MULTILINE	re.M / (?m)	^/$ match per line
re.DOTALL	re.S / (?s)	. matches newlines

Next: Real-World Recipes