Detailed explanation of the syntax of Python regular expressions with examples

In the previous article, we gave a general introduction to Python regular expressions. In fact, regular expression is a special character sequence, which can help you easily check whether a string is Matches a pattern. Python has added the re module since version 1.5, which provides Perl-style regular expression patterns. The re module brings full regular expression functionality to the Python language. The compile function generates a regular expression object based on a pattern string and optional flag arguments. This object has a series of methods for regular expression matching and replacement. The re module also provides functions that are exactly the same as these methods. These functions use a pattern string as their first parameter. This chapter mainly introduces the regular expression processing functions commonly used in Python.

String is the most involved data structure in programming, and the need to operate on string is almost everywhere. For example, to determine whether a string is a legal email address, although you can program extract the substrings before and after @, and then separately determine whether it is a word and a domain name, but this is not only troublesome, but also Code is difficult to reuse.

Regular expression is a powerful weapon used to match strings. Its design idea is to use a descriptive language to define a rule for a string. Any string that conforms to the rule is considered to "match". Otherwise, the string is illegal.

So the way we judge whether a string is a legal Email is:

1. Create a regular expression that matches Email;

2. Use this regular expression Formula to match the user's input to determine whether it is legal.

Because regular expressions are also represented by strings, we must first understand how to use characters to describe characters.

In regular expressions, if characters are given directly, it is an exact match. Use \d to match a number, \w to match a letter or number, so:

1. '00\d' can match '007', but cannot match '00A';

2.'\d\d\d' can match '010';

3.'\w\w\d' can match 'py3';

. can match any character , so:

5.'py.' can match 'pyc', 'pyo', 'py!' and so on.

To match variable-length characters, in the regular expression formula, use * to represent any number of characters (including 0), use to represent at least one character, and use ? to represent 0 or 1 characters, use {n} to represent n characters, use {n,m} to represent n-m characters:

Let’s look at a complex example: \d{3}\s \d{3,8} .

Let’s interpret it from left to right:

1.\d{3} means matching 3 numbers, such as '010';

2.\s OK Matches a space (including tab and other whitespace characters), so \s means at least one space, such as matching ' ', ' ', etc.;

3.\d{3,8} means 3-8 Number, for example '1234567'.

Taken together, the above regular expression can match phone numbers with area codes separated by any number of spaces.

What if you want to match a number like '010-12345'? Since '-' is a special character, it needs to be escaped with '\' in regular expressions, so the above regular expression is \d{3}\-\d{3,8}.

However, '010 - 12345' still cannot be matched because of spaces. So we need more complex matching methods.

Advanced

For more accurate matching, you can use [] to represent the range, such as:

1. [0-9a-zA-Z\_] can match a number, letter or underscore;

2.[0-9a-zA-Z\_] can match at least one number, letter or underscore Strings, such as 'a100', '0_Z', 'Py3000', etc.;

3.[a-zA-Z\_][0-9a-zA-Z\_]* can match Starting with a letter or an underscore, followed by any number of strings consisting of a number, letter or underscore, which is a legal variable in Python;

4.[a-zA-Z\_][0-9a -zA-Z\_]{0, 19} more precisely limits the length of the variable to 1-20 characters (up to 19 characters after the first character).

A|B can match A or B, so (P|p)ython can match 'Python' or 'python'.

^ means the beginning of the line, ^\d means it must start with a number.

$ indicates the end of the line, \d$ indicates that it must end with a number.

You may have noticed that py can also match 'python', but adding ^py$ turns it into a whole line match, so it can only match 'py'.

re module

有了准备知识，我们就可以在Python中使用正则表达式了。Python提供re模块，包含所有正则表达式的功能。由于Python的字符串本身也用\转义，所以要特别注意：

s = 'ABC\\-001' # Python的字符串
# 对应的正则表达式字符串变成：
# 'ABC\-001'

因此我们强烈建议使用Python的r前缀，就不用考虑转义的问题了：

s = r'ABC\-001' # Python的字符串
# 对应的正则表达式字符串不变：
# 'ABC\-001'

先看看如何判断正则表达式是否匹配：

>>> import re
>>> re.match(r'^\d{3}\-\d{3,8}$', '010-12345')
<_sre.SRE_Match object; span=(0, 9), match=&#39;010-12345&#39;
>>>> re.match(r&#39;^\d{3}\-\d{3,8}$&#39;, &#39;010 12345&#39;)
>>>

match()方法判断是否匹配，如果匹配成功，返回一个Match对象，否则返回None。常见的判断方法就是：

test = '用户输入的字符串'
if re.match(r'正则表达式', test):
    print('ok')
else:
    print('failed')

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