What are the rules for regular expressions

Regular expression rules include character categories, special characters, selectors, escape characters, capturing groups, backreferences, zero-width assertions, quantifiers, boundaries and comments, etc. Detailed introduction: 1. Character categories, specific character categories can be used in regular expressions to represent character sets; 2. Special characters, there are many special characters in regular expressions, used to express specific meanings; 3. Selectors, use "|" represents the selection relationship, that is, matching the subexpression on the left side of "|" or the subexpression on the right side; 4. Escape characters, use backslash "\" to escape special characters, etc.

Regular expression is a powerful text processing tool that can help us match, search and replace specific patterns in text. Regular expressions have many rules and syntax. Below I will introduce some commonly used rules in detail:

1. Character categories: Specific character categories can be used in regular expressions to represent character sets. For example, [a-z] matches all lowercase letters, [A-Z] matches all uppercase letters, and [0-9] matches all numbers. In addition, other character categories can also be used, such as [:alnum:] to match letters and numbers, [:alpha:] to match letters, etc.

2. Special characters: There are many special characters in regular expressions, which are used to express specific meanings. For example, ^ means matching the beginning of the string, $ means matching the end of the string, * means matching the previous subexpression zero or more times, means matching the previous subexpression one or more times, ? means matching the previous subexpression The expression is zero or one time, {n} means matching the previous subexpression n times, {n,} means matching the previous subexpression at least n times, {n,m} means matching the previous subexpression at least n times And no more than m times.

3. Selector: Use | to express the selection relationship, that is, match the subexpression on the left side or the subexpression on the right side of |. For example, [a-z]|[A-Z] matches lowercase letters or uppercase letters.

4. Escape characters: Use backslash\ to escape special characters, for example, to match the "" character in the text. For example, * matches characters in text.

5. Capturing group: Use parentheses () to define a capturing group. The content in the capturing group will be saved separately and can be used in subsequent expressions. For example, ([a-z] ) means match one or more lowercase letters and save them as capturing group 1.

6. Backreference: Use \number to refer to the previously captured group. For example, \1 means referencing the first capturing group. For example, ([a-z] )\1 means matching two or more identical lowercase letter sequences.

7. Zero-width assertion: Use (?:) to represent a non-capturing group, which will not save the matching content. For example, (?:[a-z] ) matches one or more lowercase letters but does not save them as a capturing group.

8. Quantifier: Use {n} to indicate that the previous subexpression matches exactly n times. Use {n,} to indicate that the previous subexpression matches at least n times. Use {n,m} to indicate that the previous subexpression matches at least n times. Indicates that the previous subexpression is matched at least n times and no more than m times. For example, [a-z]{3} means matching three consecutive lowercase letters, and [a-z]{3,5} means matching three to five consecutive lowercase letters.

9. Boundary: Use \b to represent word boundaries and \B to represent non-word boundaries. For example, \b[a-z]\b matches a complete lowercase letter word.

10. Comment: Use (?#) to indicate a comment. The comment content will not affect the matching of regular expressions. For example, (?#This is a comment).

In addition to these basic rules, regular expressions have many other rules and syntax, such as forward negative preview, backward negative preview, zero-width ignore assertion, non-greedy quantifier, reverse Repeat and so on. These rules allow us to build more complex and flexible regular expressions to handle various text patterns.

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