{"id":3195,"date":"2021-12-02T08:12:12","date_gmt":"2021-12-02T08:12:12","guid":{"rendered":"https:\/\/www.pythontutorial.net\/?page_id=3195"},"modified":"2021-12-03T08:34:22","modified_gmt":"2021-12-03T08:34:22","slug":"python-regex-backreferences","status":"publish","type":"page","link":"https:\/\/www.pythontutorial.net\/python-regex\/python-regex-backreferences\/","title":{"rendered":"Python Regex Backreferences"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you’ll learn about Python regex backreferences and how to apply them effectively.<\/p>\n\n\n\n

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

Backreferences like variables<\/a> in Python. The backreferences allow you to reference capturing groups<\/a> within a regular expression<\/a>. <\/p>\n\n\n\n

The following shows the syntax of a backreference:<\/p>\n\n\n

\\N<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Alternatively, you can use the following syntax:<\/p>\n\n\n
\\g<N><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this syntax, N<\/code> can be 1, 2, 3, etc. that represents the corresponding capturing group. <\/p>\n\n\n\n
Note that the \\g<0><\/code> refer to the entire match, which has the same value as the match.group(0)<\/code>.<\/p>\n\n\n\n
Suppose you have a string with the duplicate word Python<\/code> like this:<\/p>\n\n\n
s = 'Python Python is awesome'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
And you want to remove the duplicate word (Python<\/code>) so that the result string will be:<\/p>\n\n\n
Python is<\/span> awesome<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To do that, you can use a regular expression with a backreference.<\/p>\n\n\n\n
First, match a word with one or more characters and one or more space:<\/p>\n\n\n
'\\w+\\s+'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Second, create a capturing group that contains only the word characters:<\/p>\n\n\n
'(\\w+)\\s+'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Third, create a backreference that references the first capturing group:<\/p>\n\n\n
'(\\w+)\\s+\\1'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
In this pattern, the \\1<\/code> is a backreference that references the (\\w+<\/code>) capturing group.<\/p>\n\n\n\n
Finally, replace the entire match with the first capturing group using the sub()<\/code> function from the re<\/code> module:<\/p>\n\n\n
import<\/span> re\n\ns = 'Python Python is awesome'<\/span>\n\nnew_s = re.sub(r'(\\w+)\\s+\\1'<\/span>, r'\\1'<\/span>, s)\n\nprint(new_s)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
Python is<\/span> awesome<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
More Python regex backreference examples #<\/a><\/h2>\n\n\n\n
Let’s take some more examples of using backreferences.<\/p>\n\n\n\n
1) Using Python regex backreferences to get text inside quotes #<\/a><\/h3>\n\n\n\n
Suppose you want to get the text within double quotes:<\/p>\n\n\n
\"This is regex backreference example\"<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Or single quote:<\/p>\n\n\n
'This is regex backreference example'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
But not mixed of single and double-quotes. The following will not match:<\/p>\n\n\n
'not match\"<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To do this, you may use the following pattern:<\/p>\n\n\n
'[\\'\"](.*?)[\\'\"]'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
However, this pattern will match text that starts with a single quote (‘) and ends with a double quote (“) or vice versa. For example:<\/p>\n\n\n
import<\/span> re\n\ns = '\"Python\\'s awsome\". She said'<\/span>\npattern = '[\\'\"].*?[\\'\"]'<\/span>\n\nmatch = re.search(pattern, s)\n\nprint(match.group(0<\/span>))<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
It returns the \"Python'<\/code> not \"Python's awesome\"<\/code>:<\/p>\n\n\n
\"Python'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
To fix it, you can use a backreference:<\/p>\n\n\n
r'([\\'\"]).*?\\1'<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
The backreference \\1<\/code> refers to the first capturing group. So if the subgroup starts with a single quote, the \\1<\/code> will match the single quote. And if the subgroup starts with a double-quote, the \\1<\/code> will match the double-quote. <\/p>\n\n\n\n
For example:<\/p>\n\n\n
import<\/span> re\n\ns = '\"Python\\'s awsome\". She said'<\/span>\npattern = r'([\\'\"])(.*?)\\1'<\/span>\n\nmatch = re.search(pattern, s)\nprint(match.group())<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
\"Python's awsome\"<\/span><\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
2) Using Python regex backreferences to find words that have at least one consecutive repeated character #<\/a><\/h3>\n\n\n\n
The following example uses a backreference to find words that have at least one consecutive repeated character:<\/p>\n\n\n
import<\/span> re\n\nwords = ['apple'<\/span>, 'orange'<\/span>, 'strawberry'<\/span>]\npattern = r'\\b\\w*(\\w)\\1\\w*\\b'<\/span>\n\nresults = [w for<\/span> w in<\/span> words if<\/span> re.search(pattern, w)]\n\nprint(results)<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Output:<\/p>\n\n\n
['apple'<\/span>, 'strawberry'<\/span>]<\/code><\/span>Code language:<\/span> Python<\/span> (<\/span>python<\/span>)<\/span><\/small><\/pre>\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
  • Use a backreference \\N<\/code> to reference the capturing group N<\/code> in a regular expression.<\/li><\/ul>\n
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