Python regular expression (1)---the basis of regular expressions

1, Introduction

　　Regular expressions are powerful tools for working with strings , with their own unique syntax and an independent processing engine, which may not be as efficient as Str's own approach, but very powerful. Thanks to this, in the language that provides the regular expression, the syntax of the regular expression is the same, except that the number of grammars supported by different programming languages is different; but don't worry, the unsupported syntax is usually the less common part.

Shows the process of matching using regular expressions:

The approximate matching process for regular expressions is to take out the expression and compare the characters in the text, and if each character matches, the match succeeds; If a match is unsuccessful, the match fails. If there are quantifiers or boundaries in an expression, the process can be slightly different, but it is also well understood, with examples of fancy and a few more times you can understand them.

2,python supported regular expression meta-characters and syntax

3, the greedy mode of quantitative words and non-greedy mode

Regular expressions are typically used to find matching strings in text. The number of words in Python is greedy by default (which may be the default non-greedy in a few languages), always trying to match as many characters as possible, and not greedy, instead, always trying to match as few characters as possible. For example: the regular expression "ab*" will find "abbb" if it is used to find "ABBBC". And if you use a non-greedy quantity word "ab*?", you will find "a".

4, the anti-slash puzzle

As with most programming languages, "\" is used as an escape character in regular expressions, which can cause a backslash to be plagued. If you need to match the character "\" in the text, then 4 backslashes "\\\\" will be required in the regular expression expressed in the programming language: the first two and the last two are used to escape the backslash in the programming language, converted to two backslashes, and then escaped in the regular expression into a backslash. The native string in Python solves this problem well, and the regular expression in this example can be expressed using R "\ \". Similarly, a "\\d" that matches a number can be written as r "\d". With the native string, you no longer have to worry about missing the backslash, and the expression is more intuitive.

5, matching mode

Regular expressions provide some matching patterns that are available, such as ignoring case, multiline matching, and so on, which is described in the factory method Re.compile (pattern[, flags]) of the pattern class.

Python regular expression (1)---the basis of regular expressions