{"id":3327,"date":"2021-12-09T09:41:59","date_gmt":"2021-12-09T09:41:59","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3327"},"modified":"2021-12-09T09:46:24","modified_gmt":"2021-12-09T09:46:24","slug":"python-regex-flags","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-regex\/python-regex-flags\/","title":{"rendered":"Python Regex Flags"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about the Python regex flags and how they change the behavior of the regex engine for pattern matching.<\/p>\n\n\n\n

Introduction to the Python regex flags #<\/a><\/h2>\n\n\n\n

The regular expression functions like findall<\/a>, finditer<\/a>, search<\/a>, match<\/a>, split<\/a>, sub<\/a>, … have the parameter (flags<\/code>) that accepts one or more regex flags. <\/p>\n\n\n\n

Since Python 3.6, regex flags are instances of the RegexFlag<\/code> enumeration<\/a> class in the re<\/code> module. The following table shows the available regex flags and their meanings:<\/p>\n\n\n\n

Flag<\/th>Alias<\/th>Inline Flag<\/th>Meaning<\/th><\/tr><\/thead>
re.ASCII<\/code><\/td>re.A<\/code><\/td>?m<\/code><\/td>The re.ASCII<\/code> is relevant to the byte patterns only. It makes the \\w<\/code>, \\W<\/code>,\\b<\/code>, \\B<\/code>, \\d<\/code>, \\D, and \\S<\/code> perform ASCII-only matching instead of full Unicode matching.<\/td><\/tr>
re.DEBUG<\/code><\/td>N\/A<\/td>N\/A<\/td>The re.DEBUG<\/code> shows the debug information of compiled pattern.<\/td><\/tr>
re.IGNORECASE<\/code><\/td>re.I<\/code><\/td>?i<\/code><\/td>perform case-insensitive matching. It means that the [A-Z]<\/code> will also match lowercase letters.<\/td><\/tr>
re.LOCALE<\/code><\/td>re.L<\/code><\/td>?L<\/code><\/td>The re.LOCALE<\/code> is relevant only to the byte pattern. It makes the \\w<\/code>, \\W<\/code>, \\b<\/code>, \\B<\/code> and case-sensitive matching dependent on the current locale. The re.LOCALE<\/code> is not compatible with the re.ASCII<\/code> flag.<\/td><\/tr>
re.MUTILINE<\/code><\/td>re.M<\/code><\/td>?m<\/code><\/td>The re.MULTILINE<\/code> makes the ^<\/code> matches at the beginning of a string and at the beginning of each line and $<\/code> matches at the end of a string and at the end of each line.<\/td><\/tr>
re.DOTALL<\/code><\/td>re.S<\/code><\/td>?s<\/code><\/td>By default, the dot (.<\/code>) matches any characters except a newline. The re.DOTALL<\/code> makes the dot (.<\/code>) matches all characters including a newline.<\/td><\/tr>
re.VERBOSE<\/code><\/td>re.X<\/code><\/td>?x<\/code><\/td>The re.VERBOSE<\/code> flag allows you to organize a pattern into logical sections visually and add comments.<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

To combine two or more flags, you use the |<\/code> operator like this:<\/p>\n\n\n

re. A | re.M | re.S<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Python regex flags  #<\/a><\/h2>\n\n\n\n
Let’s take some examples of using the Python regex flags.<\/p>\n\n\n\n
1) The re.IGNORECASE flag example #<\/a><\/h3>\n\n\n\n
The following example uses the findall()<\/code> function to match all lowercase characters in the set [a-z]<\/code> in a string:<\/p>\n\n\n
import<\/span> re\n\ns = 'Python is awesome'<\/span>\npattern = '[a-z]+'<\/span>\n\nl = re.findall(pattern, s)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['ython'<\/span>, 'is'<\/span>, 'awesome'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that the letter P<\/code> is not included in the result because it is not in the set [a-z]<\/code>.<\/p>\n\n\n\n
The following example uses the re.INGORECASE<\/code> flag:<\/p>\n\n\n
import<\/span> re\n\ns = 'Python is awesome'<\/span>\npattern = '[a-z]+'<\/span>\n\nl = re.findall(pattern, s, re.IGNORECASE)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['Python'<\/span>, 'is'<\/span>, 'awesome'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Even though the pattern matches only characters in the set [a-z]<\/code>, the re.IGNORECASE<\/code> flag instructs the regex engine to also match characters in [A-Z]<\/code> set.<\/p>\n\n\n\n
2) The re.MULTILINE flag example  #<\/a><\/h3>\n\n\n\n
The following example uses the ^ anchor to match one or more word characters at the beginning of a string:<\/p>\n\n\n
import<\/span> re\n\ns = '''Regex \nFlags'''<\/span>\n\npattern ='^\\w+'<\/span>\n\nl = re.findall(pattern,s)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['Regex'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The s string has two lines. The ^<\/code> only match at the beginning of the string as expected.<\/p>\n\n\n\n
If you use the re.MULTILINE<\/code> flag, the ^<\/code> will match at the beginning of each line. For example:<\/p>\n\n\n
import<\/span> re\n\ns = '''Regex \nFlags'''<\/span>\n\npattern = '^\\w+'<\/span>\n\nl = re.findall(pattern, s, re.MULTILINE)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['Regex'<\/span>, 'Flags'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
3) The re.DOTALL flag example #<\/a><\/h3>\n\n\n\n
In this example, the dot .+<\/code> pattern match one or more characters except for the new line:<\/p>\n\n\n
import<\/span> re\n\ns = '''Regex\nFlags'''<\/span>\n\npattern = '.+'<\/span>\n\nl = re.findall(pattern, s)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['Regex'<\/span>, 'Flags'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
If you use the re.DOTALL<\/code> flag, the .+<\/code> will also match the new line:<\/p>\n\n\n
import<\/span> re\n\ns = '''Regex\nFlags'''<\/span>\n\npattern = '.+'<\/span>\n\nl = re.findall(pattern, s, re.DOTALL)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['Regex\\nFlags'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
4) The re.VERBOSE flag example #<\/a><\/h3>\n\n\n\n
The following example shows how to use the re.VERBOSE<\/code> flag to write a pattern in sections with comments:<\/p>\n\n\n
import<\/span> re\n\ns = 'Python 3'<\/span>\n\npattern = r'''^(\\w+) # match one or more characters at the beginning of the string\n               \\s*   # match zero or more spaces\n              (\\d+)$ # match one or more digits at the end of the string'''<\/span>\n\nl = re.findall(pattern, s, re.VERBOSE)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
[('Python'<\/span>, '3'<\/span>)]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the re.VERBOSE<\/code> flag allows us to add spaces and comments to the regular expression to explain each individual rule.<\/p>\n\n\n\n
5) The re.ASCII flag example #<\/a><\/h3>\n\n\n\n
The following example matches words with two characters:<\/p>\n\n\n
import<\/span> re\n\ns = '\u4f5c\u6cd5 is Pythonic in Japanese'<\/span>\npattern = r'\\b\\w{2}\\b'<\/span>\n\nl = re.findall(pattern, s)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['\u4f5c\u6cd5'<\/span>, 'is'<\/span>, 'in'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
However, if you use the re.ASCII<\/code> flag, the matches will contain only ASCII characters:<\/p>\n\n\n
import<\/span> re\n\ns = '\u4f5c\u6cd5 is Pythonic in Japanese'<\/span>\npattern = r'\\b\\w{2}\\b'<\/span>\n\nl = re.findall(pattern, s, re.ASCII)\nprint(l)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['is'<\/span>, 'in'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the word \u4f5c\u6cd5<\/code> was excluded from the resulting list.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
  • The Python regex flags are instances of the RegexFlag enumeration class.<\/li>
  • The regex flags change the way the regex engine performs pattern matching.<\/li>
  • The regex functions like match, fullmatch, findall, finditer, search, split, sub, subn accept a flags parameter that can be a flag a combination of regex flags.<\/li>
  • Use the | operator to combine two regex flags.<\/li><\/ul>\n
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