Special Characters and Pattern Matching |
Python allows you to specify a pattern to match when searching for a substring contained in a string. This pattern is known as a regular expression. For example, if you want to find a North American-style phone number in a string, you can define a pattern consisting of three digits, followed by a dash (-), and followed by four more digits.
In a regular expression, certain characters have special meanings. The table below lists these special characters and their meanings.
| Character | Meaning | Example |
|---|---|---|
| * | Zero or more occurrences of a character | ab*c matches ac, abc, abbc, and so on |
| + | One or more occurrences of a character | ab+c matches abc, abbc, and so on |
| ? | Zero or one occurrences of a character | ab?c matches ac and abc |
| . | Any character | a.*c matches any substring starting with a and ending with c |
| [chars] | Any character inside the brackets | a[bB]c matches abc and aBc |
| [char1-char2] | A range of characters | a[a-z]c matches a, followed by any non-capitalized letter, followed by c |
| [^chars] | Any character not inside the brackets | a[^bB]c matches a, followed by anything but b or B, followed by c |
| [^char1-char2] | Any character not in a range of characters | a[^a-z]c matches a, followed by anything that is not a lower-case letter, followed by c |
| {num} | An exact number of occurrences of a character | ab{3}c matches abbbc |
| {num1,num2} | A number of occurrences of a character in a specified range | ab{1,3}c matches abc, abbc and abbbc |
| | | Matches either of two alternatives | abc|aBc matches abc or aBc |
| ^ | Matches the start of the string only | ^abc matches abc in abcd, but does not match abc in dabc |
| $ | Matches the end of the string only | abc$ matches abc in dabc, but does not match abc in abcd |
| ^(?!str) | Matches anything other than the specified string | ^(?!abc) matches anything other than abc |
| ^(?!str1|str2) | Matches anything other than the two specified strings | ^(?!abc|def) matches anything other than abc and def |
If you use multiple special characters in a pattern, you can use parentheses () to specify the order in which the characters are to be interpreted.
Here is an example of a search that uses a regular expression:
import re
substring = '(iss)+'
search = re.search(substring,'Mississippi')
if search:
match = search.group()
index = search.start()
print "matched", match, "at index", index
else:
print "No match found"
|
In this example, the regular expression is (iss)+, which means "one or more occurrences of the substring iss". In the string Mississippi, this matches ississ. (It also matches iss, but the matcher tries to match as large a substring as possible.) The function group() returns the substring that was matched, and start() returns the location of the substring.
If your substring contains a character that is normally interpreted as a special character, precede the character with a backslash (\):
import re substring = '\*+A' search = re.search(substring,'***A string containing asterisks***') |
The pattern \*+A matches one or more occurrences of the * character. In the example, this matches ***A.
The backslash character is also used to define character sequences that have a special meaning in patterns. The most commonly used of these character sequences are:
- \s, which matches any whitespace character, including space, tab, newline, and carriage return;
- \d, which matches any digit character (0 through 9);
- \w, which matches any digit, any alphabetic character, or underscore.
For a complete list of the special character sequences that can be used in patterns, see the Python documentation for the re module.