regular expressions

The ^ anchor matches the character or group to its right r only at the start of a string. It does not match if r appears elsewhere.

she sells seashells

The $ anchor matches the character or group to its left s only at the end of a string. It does not match if s appears elsewhere.

r e g u l a r e x p r e s s i o n s

The . wildcard matches any single character (including spaces) but not newline characters \n. It does not match multiple characters unless combined with a quantifier like * or +.

www.example.com

The \ character is used to escape special characters (e.g., \. for a literal dot) or to denote character classes (e.g., \d for digits). See the Character Classes section for more details.

Action Button

The | (OR) operator matches either the expression to its left A or its right B, finding all possible matches across the string.

a b c b b b d b

The + quantifier greedily matches the preceding expression b one or more times. It captures the longest possible sequences of b in the string.

a b c b b b d b

The * quantifier greedily matches zero or more occurrences of the preceding expression b, including empty matches at positions where no b exists.

color colour

The ? quantifier matches the preceding character or group zero or one times, making it optional.

uuu uuuu uu u

The {m} quantifier matches the preceding character u exactly m times.

uuu uuuu uu u

The {m,n} quantifier matches the preceding character at least m times but not more than n times.

uuu uuuu uu u

The {m,n}? quantifier matches the preceding character at least m times but not more than n times, in a non-greedy (lazy) manner.

Regular Expression

Square brackets [] define a set, where each character is matched independently. A match occurs if any character from the set appears in the text.

1 Fig, 2 NewTons

The - in [m-n] is a range operator, matching any character from m to n.

a to z is not = A-Z

The \ escapes the - , treating it as a literal character instead of a range operator. This set matches a,z, and - only.

regular-expression

Matches a and the literal - because - is treated as a character when placed at the start or end of a set.

regular-expression

As above, matches a or -.

396 ExpressionS

Matches characters from a to z and also from 0 to 9.

(valid) *expressions+words

Special characters become literal inside a set, so this matches (, +, *, and ).

regular expression

The ^ negates the set, matching any character not in the set. Here, it matches characters that are not e, r, or s.

Ch4racter_Class3s

Matches all alphanumeric characters (a-z, A-Z, and 0-9). It also matches the underscore _.

! @ # $ % ^ & * ( )

Matches any non-word character, which includes symbols, punctuation, and spaces. Non-word characters are anything not in the set [a-zA-Z0-9_].

1a2b3c

Matches all digits 0-9.

1a2b3c

Matches any non-digits.

character classes

Matches whitespace characters including the \t, \n, \r, and space characters.

character classes

Matches non-whitespace characters.

character classes

Matches a word boundary, the position between a \w character (letter, digit, or underscore) and a \W character (non-word character). It doesn’t match actual characters but positions like the start or end of words.

character classes

Matches where \b does not, that is, the boundary of \w characters.

color colour

Matches the start of the string. The backslash \ escapes the normal meaning of A, turning it into a special positional anchor. Unlike ^, which matches the start of each line in multi-line mode, \A always matches the very beginning of the entire string.

color colour

Matches the end of the string. The backslash \ escapes the normal meaning of Z, turning it into a special positional anchor. Unlike $, which matches the end of each line in multiline mode, \Z always matches the very end of the entire string, excluding any trailing newline.

Finds all non-overlapping matches of the pattern A in string B and returns them as a list. If no matches are found, it returns an empty list.

Searches string B for the first occurrence of the pattern A and returns a match object. If no match is found, it returns None

Splits string B into a list at each occurrence of the pattern A If no match is found, it returns the original string as a single-element list.

Replaces all occurrences of the pattern A in string C with the string B and returns the modified string. The original string C remains unchanged.

Attempts to match the pattern A starting strictly at position 0 in string B . If the pattern doesn’t match at the start, it returns None. Unlike re.search(), it does not evaluate the rest of the string.

groups

Captures the substring ro as a group. Groups are denoted by parentheses () and can be accessed later for further processing.

cat fish dog

A non-capturing group groups patterns without creating a capturing group. Use non-capturing groups when grouping is needed for logic but you don’t need to extract the group.

catfish catdog

A positive lookahead asserts that the pattern fish must follow cat for a match. It checks the context after the current match without consuming it.

catfish dogfish

A positive lookbehind asserts that the pattern cat must precede fish for a match. It checks the context before the current match without consuming it.

catfish catdog

A negative lookahead asserts that the pattern fish must not follow cat for a match. It checks the context after the current match without consuming it.

catfish dogfish

A negative lookbehind asserts that the pattern cat must not precede fish for a match. It checks the context before the current match without consuming it.

catcat dogcat

The backreference construct \1 refers to the first captured group in the pattern. Subsequent groups can be referenced with \2, \3, and so on.

dog cat fish

The named group construct (?P...) assigns a name to the captured group for easy reference later in the regex. The P in ?P stands for Python.

dog cat fish

The named group backreference (?P=name) matches the content previously captured by the named group name. In this example, it matches the word cat.

this is a comment and will not match

The comment construct allows you to include comments in your regex. These comments are ignored by the regex engine and do not affect the match result.

The inline flag setting construct (?flags) applies one or more flags to modify the behavior of the regex pattern that follows it. Use (?flags:regex) for group matching with flags.

cat 123_ CAT

The ASCII-only flag a restricts shorthand character classes like \w, \W, \b, and \B to match only ASCII characters, excluding Unicode.

cat Cat CAT CaT

The ignore case flag i makes the pattern caseinsensitive, allowing matches regardless of capitalization.

straße cafe Éclair

The locale-dependent flag L makes shorthand character classes like \w locale-sensitive, allowing matches based on cultural or regional rules.

catdog\ncatfish

The multi-line flag m makes ^ and $ match the start and end of each line, rather than the start and end of the entire string.

cat\ndog

The dot matches all flag s allows the . character to match newline characters in addition to all other characters.

naïve cat café

The Unicode flag u makes shorthand classes like \w, \W, \b, and \B match Unicode characters. Unlike the L flag, which applies locale-specific rules, the u flag uses Unicode rules to ensure consistent matching across languages.

cat

The verbose flag x enables extended formatting by allowing spaces and comments in the pattern for improved readability. Spaces are ignored unless escaped with a backslash.