What are python regular expressions?

Python regular expression types include matching specific characters, repeated characters, selection, grouping and reference, predefined patterns, boundary conditions, greedy and non-greedy matching, etc. Detailed introduction: 1. Match specific characters: .: match any character (except newline); ^: match the beginning of the input string; $: match the end of the input string; \d: match any number, equivalent to [0- 9]; \D: matches any non-numeric characters, equivalent to [^0-9]; \s: matches any whitespace characters (including spaces, tabs, form feeds, etc.), etc.

Operating system for this tutorial: Windows 10 system, Dell G3 computer.

Regular Expression (Regular Expression) in Python is a powerful text processing tool that can match, search, replace or split complex string patterns. Here are some common Python regular expressions:

1. Matches specific characters:

   • .: Matches any character (except newline).
   • ^: Matches the beginning of the input string.
   • $: Matches the end of the input string.
   • \d: Matches any number, equivalent to [0-9].
   • \D: Matches any non-numeric characters, equivalent to [^0-9].
   • \s: Matches any whitespace characters (including spaces, tabs, form feeds, etc.).
   • \S: Matches any non-whitespace character.
   • \w: Matches any letters or numbers or underscores, equivalent to [a-zA-Z0-9_].
   • \W: Matches any non-letter, non-digit, and non-underscore characters, equivalent to [^a-zA-Z0-9_].

2. Repeating characters:

   • *: Matches the previous subexpression zero or more times.
   • : Matches the previous subexpression one or more times.
   • ?: Matches the previous subexpression zero or one time.
   • {n}: n is a non-negative integer. Match a certain number of n times.
   • {n,}: n is a non-negative integer. Match at least n times.
   • {n,m}: m and n are both non-negative integers. Match at least n times and at most m times.

3. Selection, grouping and citation:

   • |: Indicates or, for example, a|b matches 'a' or 'b'.
   • ( ): Combine several items into one unit, for example (abc) and abc match the same content. The captured content can be referenced by \1,\2,\3... etc.
   • \: Escape special characters, such as \() to match the real "(" character instead of as a grouping character.

4. Predefined pattern :

   • \d or \D: Matches one or more numeric or non-numeric characters.
   • \s or \S: Matches one or more whitespace or non-whitespace characters.
   • .: In the re module, . cannot be used directly because it is considered a special character. If you want to match any character (including newlines), you can use such as [\s\S] or [^ \s] mode.

5. Boundary conditions:

   • ^: indicates negation outside square brackets, and can also indicate the beginning of a string. Non-negative integers are represented within square brackets. For example, [0-9]^ represents a string of numbers starting with 0.
   • $: represents the end of the string and can also represent the dollar sign. Within square brackets Represents a negative integer, for example, [-1]^ represents a string of numbers ending with -1.

6. Greedy and non-greedy matching:

By default, regular expressions are greedy, that is, they match as much as possible (as long as other requirements are met). You can use ? to make a regular expression non-greedy (match as few matches as possible). For example, when looking for all words starting with "a", "a*" will match as many "a" characters as possible, and "a*?" will match only the minimum "a" characters to satisfy the condition.

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