Flags: case, multiline, dotall, verbose

Flags fix the most common 'why doesn't it match across lines?' problems and make long patterns readable.

Flags (modifiers) change how a whole pattern behaves. The common ones exist in most flavours under similar letters:

i ignore case. Python: re.IGNORECASE.
m multiline: ^ and $ match at every line start and end, not just the string ends. Python: re.MULTILINE.
s dotall: . also matches newline. Python: re.DOTALL.
x verbose: whitespace in the pattern is ignored and # starts a comment, so complex patterns can be laid out over several lines. Python: re.VERBOSE; not available in JavaScript.

Flags can also be set inside the pattern: (?i)abc, or scoped to a group as (?i:abc). Python requires global inline flags at the very start of the pattern.

JavaScript adds flags of its own:

g global: needed for matchAll and for replace to change every match. It makes the regex object stateful through lastIndex, which surprises people who reuse a /g regex with test().
y sticky: match only at lastIndex.
u Unicode mode and v (ES2024) its stricter successor with set operations in classes.
d records start and end indices for groups.

Combine flags with | in Python (re.I | re.M) or by listing letters in JavaScript (/x/gimu).

Example

import re

text = "Title\nfirst line\nSecond line"
print(re.findall(r"^\w+", text))                    # ['Title']
print(re.findall(r"^\w+", text, re.M))              # ['Title', 'first', 'Second']
print(re.findall(r"line$", text, re.M | re.I))      # ['line', 'line']
print(re.search(r"Title.*line", text))              # None: . stops at \n
print(re.search(r"Title.*line", text, re.S).group().count("\n"))   # 2

phone = re.compile(r'''
    \(?(\d{3})\)?   # area code, optional parentheses
    [\s.-]?         # separator
    (\d{3})[\s.-]?(\d{4})
''', re.VERBOSE)
print(phone.search("call (555) 123-4567").groups())  # ('555', '123', '4567')

Remember

i ignores case, m makes ^ $ per line, s lets . match newlines
Verbose mode (x) allows whitespace and comments inside the pattern
In JavaScript, g makes a regex stateful via lastIndex

Watch out for

Reusing a /g regex with test() in JavaScript and getting alternating true/false because lastIndex moved.

Check yourself

You need ^ to match at the start of every line of a log. Which flag?

Official docs: docs.python.org/3/library/re.html#flags

Example

Remember

Watch out for

Check yourself

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