regex

Regular Expressions are a powerful way to search for text.

Literal Characters:

Most letters and numbers match themselves exactly. cat matches “cat”. The Dot (.): This special character matches any single character except, usually, a newline.

Character Classes ([]):

Square brackets let you match any one character contained within them. [abc] matches “a”, “b”, or “c”. A hyphen specifies a range, so [a-z] matches any lowercase letter.

Negation ([^]):

A caret ^ at the start of a character class negates it. [^abc] matches any character except “a”, “b”, or “c”.

Quantifiers (*, +, ?):

These control how many times the preceding element can occur. . * matches zero or more occurrences (e.g., A* matches ““,”A”, “AA”, etc.). . + matches one or more occurrences (e.g., A+ matches “A”, “AA”, etc., but not ““). . ? matches zero or one occurrence (e.g., colou?r matches”color” or “colour”).

Anchors (^, $):

These match a position, not a character. . ^ matches the beginning of the string or line. . $ matches the end of the string or line.

Common Shorthands (\d, \w, \s):

These escape sequences provide quick access to common character sets. . \d matches any digit (0-9). . \w matches any word character (alphanumeric and underscore). . \s matches any whitespace character (spaces, tabs, newlines).

Alternation (|):

The pipe character acts as a logical OR. cat|dog matches either “cat” or “dog”.

Grouping and Capturing (()):

Parentheses group parts of the expression together, so quantifiers or alternation can apply to the entire group. They also create “capture groups” which allow you to extract the matched text.

Escaping (\):

To match a character that has a special meaning (like ., *, or ?), you must “escape” it with a backslash. For example, \. matches a literal period.