Regular Expressions (Regex)

Regular expressions (regex or regexp) are powerful patterns used for searching, matching, and manipulating text.

Basic Characters

• . (dot): Matches any character except a newline.

• \d: Matches any digit (0-9).

• \D: Matches any character that is not a digit.

• \w: Matches any word character (alphanumeric and underscore).

• \W: Matches any non-word character.

• \s: Matches any whitespace character (space, tab, newline).

• \S: Matches any non-whitespace character.

Quantifiers

• *: Matches 0 or more occurrences of the preceding element.

• +: Matches 1 or more occurrences of the preceding element.

• ?: Matches 0 or 1 occurrence of the preceding element.

• {n}: Matches exactly n occurrences of the preceding element.

• {n,}: Matches n or more occurrences of the preceding element.

• {n,m}: Matches between n and m occurrences of the preceding element.

Anchors

• ^: Matches the start of a line.

• $: Matches the end of a line.

Character Classes

• [abc]: Matches any single character 'a', 'b', or 'c'.

• [a-z]: Matches any lowercase letter from 'a' to 'z'.

• [A-Z]: Matches any uppercase letter from 'A' to 'Z'.

• [0-9]: Matches any digit from 0 to 9.

Predefined Character Classes

• \d is equivalent to [0-9].

• \w is equivalent to [a-zA-Z0-9_].

• \s includes various whitespace characters (space, tab, newline, etc.).

Negation

• [^abc]: Matches any character that is not 'a', 'b', or 'c'.

Grouping and Alternation

• (abc): Groups characters together.

• a|b: Matches 'a' or 'b'.

Escaping

• Some characters have special meanings in regex (e.g., .). To match them literally, escape with a backslash (e.g., \.).

Modifiers

• i: Case-insensitive matching.

• m: Multi-line matching.

• s (dotall): Allows . to match newlines.

Quantifier Greediness

• By default, quantifiers are greedy (match as much as possible). Use *?, +?, and ?? for non-greedy matching.

Greedy will consume as much as possible. Suppose you have the following:

<em>Hello World</em>

Use <.+> will get <em>Hello World</em>

But if you only want <em> or </em> , use <.+?> , the regex will match
non-greedily.

Anchors

• ^ matches the start of a line.

• $ matches the end of a line.

• \b matches a word boundary.

\bword\b  # Matches "word" as a whole word

Lookahead and Lookbehind

• (?=...): Positive lookahead assertion

  • Only find strings that have ... behind , eg:

    regex = apple(?=pie)

    applepie ✅

    apple ❌

• (?!...): Negative lookahead assertion

  • Only find strings that don't have ... behind

• (?<=...): Positive lookbehind assertion

  • Only find strings that have ... in front

• (?<!...): Negative lookbehind assertion

  • Only find strings that don't have ... in front

Comments

• (?#...): Add comments within your regular expression.