A regular expression is an exacting series of makeup that aids users to bout or discover other strings or sets of strings, using a particular syntax detained in a pattern. Regular expressions are generally used in the world of UNIX. Every Python module RE applies full support for similar Perl regular expressions. These include modules raising the exception re.error if a fault occurs when compiling a regular expression.
We would cover two essential functions, which would be utilized to handle regular expressions. But a little thing first: There are assorted characters, which would have special meaning when they are utilized in the regular expression. To confront any confusion while dealing with regular expressions, we would utilize Raw Strings as r'expression'.
This function attempts to match RE pattern to string with optional flags.
Here is the syntax for this function:
re.match(pattern, string, flags=0)
Here is the description of the parameters:
Sr.No. | Parameter & Description |
1 | pattern This is the regular expression to be matched. |
2 | string This is the string, which would be searched to match the pattern at the beginning of the string. |
3 | flags You can specify different flags using bitwise OR ('). These are modifiers, which are listed in the table below. |
The re.match function returns a match object on success, None on failure. We usegroup(num) or groups() function of the match object to get matched expression.
Sr.No. | Match Object Method & Description |
1 | group(num=0) This method returns entire match (or specific subgroup num) |
2 | groups() This method returns all matching subgroups in a tuple (empty if there weren't any) |
#!/usr/bin/python import re line = "Cats are smarter than dogs" matchObj = re.match( r'(.*) are (.*?) .*', line, re.M're.I) if matchObj: print "matchObj.group() : ", matchObj.group() print "matchObj.group(1) : ", matchObj.group(1) print "matchObj.group(2) : ", matchObj.group(2) else: print "No match!!"
When the above code is executed, it produces the following result:
matchObj.group() : Cats are smarter than dogs matchObj.group(1) : Cats matchObj.group(2) : smarter
This function searches for the first happening of RE pattern within string with optional flags.
Here is the syntax for this function-
re.search(pattern, string, flags=0)
Here is the description of the parameters:
Sr.No. | Parameter & Description |
1 | pattern This is the regular expression to be matched. |
2 | string This is the string, which would be searched to match the pattern anywhere in the string. |
3 | flags You can specify different flags using bitwise OR ('). These are modifiers, which are listed in the table below. |
The re.search function returns a match object on success, none on failure. We use group(num) or groups() function of the match object to get matched expression.
Sr.No. | Match Object Methods & Description |
1 | group(num=0) This method returns the entire match (or specific subgroup num) |
2 | groups() This method returns all matching subgroups in a tuple (empty if there weren't any) |
#!/usr/bin/python import re line = "Cats are smarter than dogs"; searchObj = re.search( r'(.*) are (.*?) .*', line, re.M're.I) if searchObj: print "searchObj.group() : ", searchObj.group() print "searchObj.group(1) : ", searchObj.group(1) print "searchObj.group(2) : ", searchObj.group(2) else: print "Nothing found!!"
When the above code is executed, it produces the following result:
searchObj.group() : Cats are smarter than dogs searchObj.group(1) : Cats searchObj.group(2) : smarter
Python, the programming language applies two assorted primeval operations. These operations are based on regular expressions: match verifies for about only at the starting of the string when search evaluates for a match anyplace in the string.
#!/usr/bin/python import re line = "Cats are smarter than dogs"; matchObj = re.match( r'dogs', line, re.M're.I) if matchObj: print "match --> matchObj.group() : ", matchObj.group() else: print "No match!!" searchObj = re.search( r'dogs', line, re.M're.I) if searchObj: print "search --> searchObj.group() : ", searchObj.group() else: print "Nothing found!!"
When the above code is executed, it produces the following result:
No match!! search --> searchObj.group() : dogs
One of the most important re methods that use regular expressions is sub.
re.sub(pattern, repl, string, max=0) #!/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
Regular expression literals may incorporate an obligatory modifier to control a variety of aspects of matching. The modifiers are specified as an optional flag. You can supply multiple modifiers utilizing exclusive OR ('), as shown previously and may be represented by one of these:
Sr.No. | Modifier & Description |
1 | re.I Performs case-insensitive matching. |
2 | re.L Interprets words according to the current locale. This interpretation affects the alphabetic group (\w and \W), as well as word boundary behavior(\b and \B). |
3 | re.M Makes $ match the end of a line (not just the end of the string) and makes ^ match the start of any line (not just the start of the string). |
4 | re.S Makes a period (dot) match any character, including a newline. |
5 | re.U Interprets letters according to the Unicode character set. This flag affects the behavior of \w, \W, \b, \B. |
6 | re.X Permits "cuter" regular expression syntax. It ignores whitespace (except inside a set [] or when escaped by a backslash) and treats unescaped # as a comment marker. |
Except for control characters, (+ ? . * ^ $ ( ) [ ] { } ' \), all characters match themselves. You can escape a control character by preceding it with a backslash.
The following table lists the regular expression syntax that is available in Python-
Sr.No. | Pattern & Description |
1 | ^ Matches beginning of the line. |
2 | $ Matches end of the line. |
3 | . Matches any single character except newline. Using the m option allows it to match the newline as well. |
4 | [...] Matches any single character in brackets. |
5 | [^...] Matches any single character not in brackets |
6 | re* Matches 0 or more occurrences of the preceding expression. |
7 | re+ Matches 1 or more occurrence of the preceding expression. |
8 | re? Matches 0 or 1 occurrence of the preceding expression. |
9 | re{ n} Matches exactly n number of occurrences of the preceding expression. |
10 | re{ n,} Matches n or more occurrences of the preceding expression. |
11 | re{ n, m} Matches at least n and at most m occurrences of the preceding expression. |
12 | a' b Matches either a or b. |
13 | (re) Groups regular expressions and remembers matched text. |
14 | (?imx) Temporarily toggles on i, m, or x options within a regular expression. If in parentheses, only that area is affected. |
15 | (?-imx) Temporarily toggles off i, m, or x options within a regular expression. If in parentheses, only that area is affected. |
16 | (?: re) Groups regular expressions without remembering matched text. |
17 | (?imx: re) Temporarily toggles on i, m, or x options within parentheses. |
18 | (?-imx: re) Temporarily toggles off i, m, or x options within parentheses. |
19 | (?#...) Comment. |
20 | (?= re) Specifies position using a pattern. Doesn't have a range. |
21 | (?! re) Specifies position using pattern negation. Doesn't have a range. |
22 | (?> re) Matches independent pattern without backtracking. |
23 | \w Matches word characters. |
24 | \W Matches nonword characters. |
25 | \s Matches whitespace. Equivalent to [\t\n\r\f]. |
26 | \S Matches non-whitespace. |
27 | \d Matches digits. Equivalent to [0-9]. |
28 | \D Matches nondigits. |
29 | \A Matches the beginning of the string. |
30 | \Z Matches end of the string. If a newline exists, it matches just before the newline. |
31 | \z Matches end of the string. |
32 | \G The matches are the point where the last match finished. |
33 | \b Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets. |
34 | \B Matches nonword boundaries. |
35 | \n, \t, etc. Matches newlines, carriage returns, tabs, etc. |
36 | \1...\9 Matches nth grouped subexpression. |
37 | \10 Matches nth grouped subexpression if it matched already. Otherwise refers to the octal representation of a character code. |
Sr.No. | Example & Description |
1 | python Match "python". |
Sr.No. | Example & Description |
1 | [Pp]ython Match "Python" or "python" |
2 | rub[ye] Match "ruby" or "rube" |
3 | [aeiou] Match any one lowercase vowel |
4 | [0-9] Match any digit; same as [0123456789] |
5 | [a-z] Match any lowercase ASCII letter |
6 | [A-Z] Match any uppercase ASCII letter |
7 | [a-zA-Z0-9] Match any of the above |
8 | [^aeiou] Match anything other than a lowercase vowel |
9 | [^0-9] Match anything other than a digit |
Sr.No. | Example & Description |
1 | . Match any character except newline |
2 | \d Match a digit: [0-9] |
3 | \D Match a nondigit: [^0-9] |
4 | \s Match a whitespace character: [ \t\r\n\f] |
5 | \S Match nonwhitespace: [^ \t\r\n\f] |
6 | \w Match a single word character: [A-Za-z0-9_] |
7 | \W Match a nonword character: [^A-Za-z0-9_] |
Sr.No. | Example & Description |
1 | ruby? Match "rub" or "ruby": they is optional |
2 | ruby* Match "rub" plus 0 or more ys |
3 | ruby+ Match "rub" plus 1 or more ys |
4 | \d{3} Match exactly 3 digits |
5 | \d{3,} Match 3 or more digits |
6 | \d{3,5} Match 3, 4, or 5 digits |
Does this match the smallest number of repetitions-
Sr.No. | Example & Description |
1 | <.*> Greedy repetition: matches "perl>" |
2 | <.*?> Nongreedy: matches "" in "perl>" |
Sr.No. | Example & Description |
1 | \D\d+ No group: + repeats \d |
2 | (\D\d)+ Grouped: + repeats \D\d pair |
3 | ([Pp]ython(, )?)+ Match "Python", "Python, python, python", etc. |
This matches a previously matched group again ?
Sr.No. | Example & Description |
1 | ([Pp])ython&\1ails Match python&pails or Python&Pails |
2 | (['"])[^\1]*\1 Single or double-quoted string. \1 matches whatever the 1st group matched. \2 matches whatever the 2nd group matched, etc. |
Sr.No. | Example & Description |
1 | python'perl Match "python" or "perl" |
2 | rub(y'le)) Match "ruby" or "ruble" |
3 | Python(!+'\?) "Python" followed by one or more ! or one |
This needs to specify the match position.
Sr.No. | Example & Description |
1 | ^Python Match "Python" at the start of a string or internal line |
2 | Python$ Match "Python" at the end of a string or line |
3 | \APython Match "Python" at the start of a string |
4 | Python\Z Match "Python" at the end of a string |
5 | \bPython\b Match "Python" at a word boundary |
6 | \brub\B \B is nonword boundary: match "rub" in "rube" and "ruby" but not alone |
7 | Python(?=!) Match "Python", if followed by an exclamation point. |
8 | Python(?!!) Match "Python", if not followed by an exclamation point. |
Sr.No. | Example & Description |
1 | R(?#comment) Matches "R". All the rest is a comment |
2 | R(?i)uby Case-insensitive while matching "uby" |
3 | R(?i:uby) Same as above |
4 | rub(?:y'le)) Group only without creating \1 backreference |
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