Challenges

Match a valid IPv4 address — four numbers 0 to 255, separated by dots, and nothing else.

The boss. Is the black king attacked by anything at all — pawn, knight, bishop, rook, queen or the white king?

Extract every double-quoted string, including its quotes, where a backslash escapes the next character.

Two piles of matchsticks. Each turn you take as many as you like from one pile, and whoever takes the last match loses. Match the positions that are already lost for the player about to move.

A 3×3 patch of Conway's Game of Life. Match it when the centre cell is alive in the next generation.

You are given a string of 1s. Match it only when its length is a composite number — that is, not prime and not 1.

Eighty-one cells. Match the grid only when no digit repeats in any row, any column, or any 3×3 box.

Extract every white bishop that is giving check along a diagonal, in board order.

A Sudoku box, flattened. Extract every digit that appears again later in the box.

The same 7x6 Connect Four board, but now a win counts in any direction — across, down, or either diagonal. Capture the winning colour, or nothing.

Extract every number that is not immediately followed by "px".

Extract every number that is not a dollar amount — that is, not preceded by a $.

Star out everything after the sk_live_ prefix, however long the key is, leaving other text untouched.

Does a white rook attack the black king along a rank or a file? Any piece standing between them blocks it.

A hand is five cards as "2H 3S 4D 5C 6H" — rank then suit, space separated, and always sorted by rank, ascending. The rank ladder is A 2 3 4 5 6 7 8 9 T J Q K: the ace is low, so A-2-3-4-5 is a run and A-T-J-Q-K is not. Match a hand whose five ranks are consecutive on that ladder.

Extract the amounts that are prefixed with £ — and not the £ itself.

Match passwords of at least 8 characters that contain at least one digit and at least one capital letter.

Match a line that does not contain the word "ERROR" anywhere in it.

Insert commas as thousands separators: 1234567 becomes 1,234,567.

Squash every run of the same repeated character down to one of it.

Match any string containing the same word character twice in a row.

Extract every run of 1 to 4 digits — but never a fragment of a longer number.

Does a white knight attack the black king? Eight fixed jumps, and nothing can block a knight.

Match a well-formed hand: exactly five cards, single space separated, each a rank from A23456789TJQK followed by a suit from SHDC — and no card appearing twice. Hands are always sorted by rank, ascending.

Match text containing a quoted section — opened and closed by the same quote character, single or double.

A hand is five cards as "AS 7H 7D 9C KS" — rank then suit, space separated, and always sorted by rank, ascending. Capture the rank that appears more than once, or match nothing if every rank is different.

Match a noughts and crosses board that is a draw: every square filled, and neither player with three in a row.

Match text where the same word appears twice in a row.

Capture the major, minor and patch numbers of a version, plus its prerelease tag — or nothing in group 4 when there is no prerelease.

A Sudoku row is nine characters. Capture the digit that appears twice — and match nothing at all when the row is legal.

A 5x5 Minesweeper grid with one square marked ?. Match it only when that square has a mine in one of its eight neighbours.

Match strings containing "cat" or "dog" as a whole word.

Validate the short two-field FEN: eight ranks separated by slashes, then a space and the active colour.

A Connect Four board, 7 wide and 6 tall, arrives as one flat string. Capture the colour — R or Y — that has four in a row horizontally. Capture nothing if neither has.

A noughts and crosses board arrives as one flat string. Match it only when X has three in a row — across, down, or diagonally.

Match a valid CSS hex colour — a # followed by exactly three or six hex digits, and nothing else.

A hand is five cards as "AS 2H 5D 9C KS" — rank then suit, space separated. Ranks are A23456789TJQK (T is ten), suits are SHDC. Capture the suit if all five cards share one; no match if they do not.

Rewrite a log line "DATE TIME LEVEL MESSAGE" as "[LEVEL] MESSAGE (TIME on DATE)".

A white pawn checks the black king from the square down-left or down-right of it. Capture both squares.

Capture the year, month and day of an ISO date as three separate groups.

Capture the key and the value from a "key = value" line, whatever the spacing.

Pull the timestamp, the level and the message out of a bracketed log line.

Extract the fields of a CSV row, where a quoted field may contain commas. Quoted fields keep their quotes.

Turn "Ada Lovelace" into "Lovelace, Ada".

Capture the text inside the FIRST <b>...</b> tag on the line — just the text, not the tags.

Rewrite every DD/MM/YYYY date as YYYY-MM-DD, leaving the rest of the text alone.

From a config file, extract every whole line that is commented out — every line beginning with a #. Inline comments on the end of a real setting do not count.

Match strings made up entirely of digits, with at least one digit.

Match strings that begin with "http".

Capture everything before the FIRST equals sign: key=value=other gives key.

Capture everything between the opening and closing --- markers of a Markdown front-matter block.

Extract "cat" only where it stands as a whole word.

Match strings that end with whitespace — the ones your linter shouts about.

Match both the British and American spellings of "colour".

Extract every http and https URL from a block of text, and nothing else that looks like a scheme.

Extract every run of five or more lower-case letters.

Collapse every run of whitespace down to a single space.

Remove the thousands separators from a number: 1,234,567.89 becomes 1234567.89.

Match strings containing three consecutive digits.

Match any string containing at least one digit.

Capture the file extension of a filename: report.q3.pdf gives pdf, not q3.pdf.

Match any string containing whitespace — a space, a tab or a newline.

Match any string that contains a character which is not an A–Z letter.

Turn snake_case into kebab-case: user_first_name becomes user-first-name.

Pull out every lower-case vowel, in order.

Match "error" in any mix of upper and lower case.

Match any string that contains the word "regex".

Match strings containing "a.b" — with a real full stop between them.