Regex

Regex will arbitrarily match pattern in a line unless pinned to a location. Any meta-character to be used literally must be escaped with a \ prepending it. Generally find what excludes the bad case.

Groups can be nested. Groups will only capture what is enclosed in (). Groups can typically be referenced with \{some_num} (like \0 or \1). Groups are referenced from \1 onwards; \0 is the whole matched string.

| can decide what to OR a little arbitrarily. Maybe on whitespace as well as group? All set matchers (like \d) can be inversed through capitalization.

Flags can be combined like (?im).

• \d will match any digit [0-9].
• \w will match any alphanumeric [A-Za-z0-9_].
• \s will match any whitespace.

• [] encapsulating a collection of characters indicates an OR.
• - can be used inside [] to indicate a sequential range of values to be represented: [a-z].
• {} can be appended to a character to indicate number of instances to match: {4}.
• , can be used in some engines inside {} to designate a range: {1,5}.
• () will capture characters match in a group for later use.
• ?: inside () will result in a non-capturing group: (?:.*).

• ^ is a meta-character IN [] for NOT.
• ^ is a meta-character OUTSIDE of [] for start of line.
• $ is a meta-character for end of line.
• \b is a boundary between alphanumeric and non-alphanumeric character.

• . is a meta-character for literally anything (including \n with (?s)).
• * is a meta-character for 0 or more matches.
• + is a meta-character for 1 or more matches.
• ? after QUANTIER (*, +, {d+,d+}) is a meta-character for laziness.
• ? after REGULAR CHARACTER/TOKEN is a meta-character for optionality.
• | indicates an OR for the whole sequence.

• (?i) flag for case-insensitive.
• (?m) flag for multiline (^ and $ match line boundaries).
• (?s) flag for dotall, making . match newlines.