The regular expression is a popular topic in system administrators and developers. A regular expression is used to find structured text or string in single or multiple files. The best side of regular expression we can define whatever we want to match string in texts. Python supports and provides a lot of methods for regular expressions and related operations. In this tutorial, we will look at these regex functions in detail.
Import Re or Regular Expression Library
In order to work with regular expressions in python, we need to import regular expression library which is named as a shortcut of
regular expression as
The match function is one of the most popular functions which will apply regex pattern into the given string. We will use
match function with
string parameters. There is also
flags parameter which can be used to provide some flags like the case, interpretation, etc. If we do not provide
flags there will be no error.
In this example, we want to find words that are delimited by spaces in the given string. Each word provides single match and those matches will be grouped.
line="This is an example about regular expression" matches = re.match('\w+',line) matches.group(0)
In the previous part, we have simply printed the first group which index is
0 but we may have more than one word to match in a line. It is called a group in the regex. We can match multiple different patterns in a single match.
In this example we will match words starts with
a into two groups.
line="This is an example about regular expression" matches = re.match('(T\w+).*example\s(a\w+)',line) matches.group(0) #'This is an example about' matches.group(1) #'This' matches.group(2) #'about'
As we see matched pattern results are assigned into groups. We can get them by providing an index about these groups.
Search is similar to the match function but the main difference is match looks up to the first match and then stops but the search will look at to the end of the string and will find multiple matches if exists. The syntax of the
search function is the same
line="This is an example about regular expression" matches = re.search('(T\w+).*example\s(a\w+)',line) matches.group(0) #'This is an example about' matches.group(1) #'This' matches.group(2) #'about'
Search and Replace
Python regex functions support finding given text and replacing the text with a new one. We will use
sub functions in order to replace.
sub function supports the following syntax.
We will change
regular word with
unregular word in this example.
line="This is an example about regular expression" matches = re.sub('regular','unregular',line) print(matches)
Options flags generally provided as last parameter to the related regex functions. Option flags generally used to case-insensitive match, interpret with current locale etc. Here is a list of option flags.
re.Iis used case-insensitive match
re.Lis used for current locale
$match end of line
.match any character, including newline
We can use option flags in order to make case-insensitive match or search with regular expression. We will provide
re.I as last arguments to the relevant function like below.
matches = re.sub('regular','unregular',line,re.I)