Lessons · Regex · flags: i, m, s and x

Switches on the whole pattern

i ignores case, m makes ^ and $ match on every line, s lets the dot take newlines, x gives the pattern room to breathe. A flag applies to the WHOLE pattern.

Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.

What it is for

Case-insensitive search, reading a whole file as one string, and making a long pattern something a colleague can read.

How to think about it

Ask what the pattern needs to stop being strict about: case, line ends, the newline itself, or its own layout. Turn on exactly those and nothing else.

Worked example

pattern: (?i)error

The flag written inside the pattern, at the front.

text: Error: disk. error: net

Both match, whatever the case.

in Python: re.findall(r'error', text, re.I)

The same switch, handed to the call instead.

Your turn

Match 'error' whatever its case, with the flag written inside the pattern.

(?)error

The trap

There is no 'find them all' flag in Python: findall already returns every match and re.sub already replaces every one unless you pass count. Reaching for a g is the commonest habit a JavaScript regex brings across.

Practise flags: i, m, s and x on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.