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Today, I will talk to you about how to analyze Python regular expressions. Many people may not know much about it. In order to make you understand better, the editor has summarized the following for you. I hope you can get something according to this article.

Today, when I wrote a crawler, I accidentally thought of learning regular expressions. I read an article that was very good. I did a search and found it.

Re module

Re.search

The form match = re.search (pat, str) is often used. Because there may not be a match, re.search () is usually followed by if statement.

Re.match

Re.match is similar to re.search, except that re.match matches from the beginning of the string.

The meaning of commonly used regular characters

A, X, 9, and other characters match themselves, metacharacters do not match themselves, because they have special meanings, such as. ^ $* +? {} []\ | ()

. English full stop, matches any character, does not contain'\ n'

\ w matches the 'word' character, [a-zA-Z0-9]

\ W matches non 'word' characters

\ b match the boundary between 'word'' and 'non-word'

\ s matches a single whitespace character, space, newline, return, tab, form [\ n\ r\ t\ f]

\ s matches non-whitespace characters

\ t,\ n,\ r match tab, newline, return

\ d matching numbers [0-9]

^ matches the beginning of the string

Match the end of the string

Repetition

'+' one or more times,'* 'zero or more times,'?' Zero or once

Square brackets []

[] similar to or

Square brackets can be used to indicate a set of chars, so [abc] matches'a 'or 'b'or 'cages.

Group Extraction parentheses ()

Sometimes we need to extract part of the matching characters, such as the mailbox just now, where we may need username and hostname. At this time, we can wrap username and hostname with (), just like r' ([\ w. -] +) @ ([\ w. -] +)'. If the match is successful, then the pattern will not change, but we can use match.group (1) and match.group (2) to keep the username and hostname,match.group () results unchanged.

Findall and groups

() is combined with findall () and returns a list of tuples if one or more group is included.

It's the same after adding ^ to re.search.

Re.sub

Re.sub (pat, replacement, str) looks in str for a string that matches pattern, and then replaces it with replacement. Replacement can contain\ 1 or\ 2 to replace the corresponding group, and then implement a local replacement.

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