Understanding Python Regular Expressions

Python: Regular Expressions

re Module Provides support of regular expressions Very similar to Perl support of regular expression Character sets Anchors Modifiers Extended RE Pattern match Pattern substitution 2

Pattern Match re.match(pattern, string, [flag]) Try to match pattern at beginning of string Return a match object if successful, None otherwise re.search(pattern, string, [flag]) Try to match pattern anywhere in the string Return a match object if successful, None otherwise str = hello, world m1 = re.match(r he , str) m2 = re.match(r (.)\1 , str) # match found # match not found m3 = re.search(r he , str) m4 = re.search(r (.)\1 , str) # match found # match found 3

Match Object Method group([num]) group(0) also returns the entire match group(n) returns the nth parenthesized match subgroup group() returns all matched subgroups, same as group(0) str = hello, world m1 = re.search(r he , str) if (m1): print(m1.group(0)) m2 = re.search(r (.)\1 , str) If (m2): print(m2.group(0)) print(m2.group(1)) See examples/python/p5/example1.py 4

Pattern Match Flags re.I Case-insensitive matching re.search(pattern, str, re.I) re.S Make period (.) match any characters, including newline re.X Ignore whitespaces and treats unescaped # as comment marker re.search( \w+ @ \w+ , str, re.X) Multiple flags can be provided with exclusive OR | re.search(pattern, string, re.S|re.X) 5

Search and Replace re.sub(pattern, replacement, string, max=0) Search for pattern in string, and replace it with replacement whenever there is a match Max specifies the maximum number of replacement (replace all if unspecified) Return resulting string phone = 2004-959-559 # comments Num = re.sub(r #.*$ , , phone) Num = re.sub(r \D , , phone) See examples/python/p5/example2.py 6

Split String using Regular Expression re.split(pattern, string, maxsplit=0, flags=0) Split string using pattern Returning a list of strings between pattern matches str = this is a test re.split(r \W+ , str) # list ['this', 'is', 'a', 'test'] See examples/python/p5/example3.py 7

Find All Occurrence of Pattern re.findall(pattern, string, flag = 0) Return all non-overlapping matches of pattern in string str = this is a test re.findall(r \w+ , str) # list ['this', 'is', 'a', 'test'] # note \w 8

Compiled Regular Expression re.compile(pattern, flags=0) Compile regular expression pattern into regular expression object, which can be used to call re methods prog = re.compile(r (.)\1 , re.I) prog.search(str) re.search(r (.)\1 , str, re.I) 9