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The characters * and ? have different meanings in regular expressions than they do in shells. Confusingly similar, but different.

Character classes are characters listed inside square brackets. For example, [aeiou] is a character class that matches any vowel.

Very important to keep in mind: different rules apply inside versus outside character classes.

Why can't grep find negative numbers? johndcook.com/blog/2019/12/0…

Inside a character class, dash means range. For example, [0-9] matches any digit. Outside a character class dash has no special meaning.

Convert a regular expression to a finite automaton: en.wikipedia.org/wiki/Thompson%…

Convert a finite automaton to a regular expression: en.wikipedia.org/wiki/Kleene%27…

Inside a character class, period and question mark are not special. For example, [.?] matches a period or a question mark.

Searching Greek and Hebrew with regular expressions johndcook.com/blog/2020/09/2…

Inside a character class, ^ means 'not'. For example, [^abc] matches any character except a, b, or c.

Outside a character class, ^ means 'start at the beginning of a line'. Example: /^The/ matches lines that begin with 'The'.

Using regular expressions to search genetic data sciencedirect.com/science/articl…

$ anchors a regular expression at the end of a strings. Example: /end$/ matches strings ending in 'end'.

(When a string contains newline characters, whether $ may match the end of the line or the end of the string depends on regex settings.)

Regular expressions and successive approximation johndcook.com/blog/2022/04/0…

\s matches white space, i.e. space, tab, newline, etc. Example: /foot\s+ball/ matches 'foot ball' but not 'football'.

\S matches anything but white space, i.e. \S is the opposite of \s. This is a common pattern with upper case vs lower case.

\d matches any digit, i.e. [0-9]. \D matches anything except a digit, i.e. [^0-9].

grep Flags – The Good Stuff zwischenzugs.com/2022/02/02/gre…

How to use regular expressions in Pandas kanoki.org/2019/11/12/how…