Совмещение r и python: зачем, когда и как?

Regular Expression Modifiers: Option Flags

Regular expression literals may include an optional modifier to control various aspects of matching. The modifiers are specified as an optional flag. You can provide multiple modifiers using exclusive OR (|), as shown previously and may be represented by one of these −

Search and Replace

One of the most important re methods that use regular expressions is sub.

Syntax

re.sub(pattern, repl, string, max=0)

This method replaces all occurrences of the RE pattern in string with repl, substituting all occurrences unless max provided. This method returns modified string.

Example

#!/usr/bin/python
import re

phone = "2004-959-559 # This is Phone Number"

# Delete Python-style comments
num = re.sub(r'#.*$', "", phone)
print "Phone Num : ", num

# Remove anything other than digits
num = re.sub(r'\D', "", phone)    
print "Phone Num : ", num

When the above code is executed, it produces the following result −

Phone Num :  2004-959-559
Phone Num :  2004959559

Sets

A set is a set of characters inside a pair of square brackets with a special meaning:

Regular Expression Patterns

Except for control characters, (+ ? . * ^ $ ( ) { } | \), all characters match themselves. You can escape a control character by preceding it with a backslash.

Following table lists the regular expression syntax that is available in Python −

Поиск и замена

Модуль повторного Python предоставляет re.sub на матч замены строки.

Синтаксис:

re.sub(pattern, repl, string, max=0)

Возвращаемая строка это строка с крайней левой RE матчей не будет повторяться, чтобы заменить. Если шаблон не найден, то символы будут возвращены без изменений.

Необязательный счетчик параметр является максимальное количество раз замену соответствующий шаблон, счетчик должен быть неотрицательным целым числом. Значение по умолчанию 0 означает, что для замены всех вхождений.

Пример:

#!/usr/bin/python
import re

phone = "2004-959-559 # This is Phone Number"

# Delete Python-style comments
num = re.sub(r'#.*$', "", phone)
print "Phone Num : ", num

# Remove anything other than digits
num = re.sub(r'\D', "", phone)    
print "Phone Num : ", num

Phone Num :  2004-959-559
Phone Num :  2004959559

Шаблоны, соответствующие не конкретному тексту, а позиции

Отдельные части регулярного выражения могут соответствовать не части текста, а позиции в этом тексте.
То есть такому шаблону соответствует не подстрока, а некоторая позиция в тексте, как бы «между» буквами.

Простые шаблоны, соответствующие позиции

всем текстомстрочкой текста

^

^Привет

$

Будь здоров!$

\A

\Z

\b

\bвал

\B

\Bвал

Special Sequences

A special sequence is a followed by one of the characters in the list below, and has a special meaning:

re.compile

With the re.compile() function we can compile pattern into pattern objects,
which have methods for various operations such as searching for pattern matches
or performing string substitutions.

Let’s see two examples, using the re.compile() function.

The first example checks if the input from the user contains only letters,
spaces or . (no digits)

Any other character is not allowed.

The second example checks if the input from the user contains only numbers,
parentheses, spaces or hyphen (no letters)

Any other character is not allowed

The output of above script will be:

Please, enter your phone: s

Please enter your phone correctly!

It will continue to ask until you put in numbers only.

Specify Pattern Using RegEx

To specify regular expressions, metacharacters are used. In the above example, and are metacharacters.

MetaCharacters

Metacharacters are characters that are interpreted in a special way by a RegEx engine. Here’s a list of metacharacters:

[] . ^ $ * + ? {} () \ |

— Square brackets

Square brackets specifies a set of characters you wish to match.

Here, will match if the string you are trying to match contains any of the , or .

You can also specify a range of characters using inside square brackets.

• is the same as .
• is the same as .
• is the same as .

You can complement (invert) the character set by using caret symbol at the start of a square-bracket.

• means any character except a or b or c.
• means any non-digit character.

— Period

A period matches any single character (except newline ).

— Caret

The caret symbol is used to check if a string starts with a certain character.

— Dollar

The dollar symbol is used to check if a string ends with a certain character.

— Star

The star symbol matches zero or more occurrences of the pattern left to it.

— Plus

The plus symbol matches one or more occurrences of the pattern left to it.

— Question Mark

The question mark symbol matches zero or one occurrence of the pattern left to it.

— Braces

Consider this code: . This means at least n, and at most m repetitions of the pattern left to it.

Let’s try one more example. This RegEx matches at least 2 digits but not more than 4 digits

— Alternation

Vertical bar is used for alternation ( operator).

Here, match any string that contains either a or b

— Group

Parentheses is used to group sub-patterns. For example, match any string that matches either a or b or c followed by xz

— Backslash

Backlash is used to escape various characters including all metacharacters. For example,

match if a string contains followed by . Here, is not interpreted by a RegEx engine in a special way.

If you are unsure if a character has special meaning or not, you can put in front of it. This makes sure the character is not treated in a special way.

Special Sequences

Special sequences make commonly used patterns easier to write. Here’s a list of special sequences:

— Matches if the specified characters are at the start of a string.

— Matches if the specified characters are at the beginning or end of a word.

— Opposite of . Matches if the specified characters are not at the beginning or end of a word.

— Matches any decimal digit. Equivalent to

— Matches any non-decimal digit. Equivalent to

— Matches where a string contains any whitespace character. Equivalent to .

— Matches where a string contains any non-whitespace character. Equivalent to .

— Matches any alphanumeric character (digits and alphabets). Equivalent to . By the way, underscore is also considered an alphanumeric character.

— Matches any non-alphanumeric character. Equivalent to

— Matches if the specified characters are at the end of a string.

Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101. This tool not only helps you in creating regular expressions, but it also helps you learn it.

Now you understand the basics of RegEx, let’s discuss how to use RegEx in your Python code.