Greedily match at least m and at most n occurrences of expression e Nongreedily match at most n occurrences of expression e Greedily match at most n occurrences of expression e Nongreedily match at least m occurrences of expression e Greedily match at least m occurrences of expression e Match exactly m occurrences of expression e Nongreedily match zero or more occurrences of expression e Greedily match zero or more occurrences of expression e Nongreedily match one or more occurrences of expression e Greedily match one or more occurrences of expression e The special characters are \.^$?+* Greedily match one or more occurrences of expression e So must be escaped by preceding them with a backslash ( \) to use them as literals. For example, the regex tune consists of four expressions, each implicitly quantified to match once, so it matches one t followed by one u followed by one n followed by one e, and hence matches the strings tune and at tuned.Īlthough most characters can be used as literals, some are “special characters”-these are symbols in the regex language and The simplest expressions are just literal characters, such as a or 5, and if no quantifier is explicitly given it is taken to be “match one occurrence”.
Shows how to use the language’s assertions and flags to affect how regular expressions work. The third subsection shows how to group subexpressions and how to capture matching text, and the final subsection The second subsection shows how to quantify matches, for example, match once, or match at least once, or match as many timesĪs possible. The first subsection shows how to match individualĬharacters or groups of characters, for example, match a, or match b, or match either a or b. In this section we look at the regular expression language in four subsections. We indicate regular expressions in the text using bold, show where they match using underlining, and show captures using shading. The complete regex language offered by the re module, including all the assertions and flags. Who just want to learn how they work in Python could skip to the second section (starting on page 455). Readers familiar with regular expressions Of Python programming, drawing on all the material covered in the earlier sections. Then the second section shows how to use regular expressions in the context Syntax-it makes minimal reference to Python itself. This chapter’s first section introduces and explains all the key regular expression concepts and shows pure regular expression MoreĬomplex regexes consist of any number of quantified expressions and may include assertions and may be influenced by flags. Splitting strings: splitting a string at each place the regex matches, for example, splitting everywhere “: ” or “=” is encounteredĪt its simplest a regular expression is an expression (e.g., a literal character), optionally followed by a quantifier.Vehicle” and replacing either with “bike” Searching and replacing: replacing everywhere the regex matches with a string, for example, finding “bicycle” or “human powered.Or “pet.svg” while avoiding “carpet.png” and similar Searching: locating substrings that can have more than one form, for example, finding any of “pet.png”, “pet.jpg”, “pet.jpeg”,.Validation: checking whether a piece of text meets some criteria, for example, contains a currency symbol followed by digits.Is completely different from Python-but Python includes the re module through which we can seamlessly create and use regexes. Regular expressions (which we will mostly call “regexes” from now on) are defined using a mini-language that Is that a single regular expression can represent an unlimited number of strings-providing they meet the regular expression’s What makes regular expressions so powerful A regular expression is a compact notation for representing a collection of strings.