Strings¶

Usage¶

String objects are very common in Python. They are especially usefull when reading and writting text files.

They are defined between simple quotes (') or double quotes (").

Caution! Opening and closing quotes must be the same

Special characters¶

Python contains a set of predefined characters, which are listed below (source: python.org)

Character	Definition
`\a`	ASCII Bell (BEL)
`\b`	ASCII Backspace (BS)
`\f`	ASCII Formfeed (FF)
`\n`	ASCII Linefeed (LF)
`\r`	ASCII Carriage Return (CR)
`\t`	ASCII Horizontal Tab (TAB)
`\v`	ASCII Vertical Tab (VT)

In order to escape these special characters, add r before the 1st quote:

str1 = '@this is \n@'  # \n interpreted as line break
print(str1)
str2 = r'%this is \n%'
print(str2)

@this is 
@
%this is \n%

String manipulation¶

String objects have a lot of methods to manipulate them.

Since they are immutable, these methods return new string objects, compared to list methods, which change the list content.

Extracting characters¶

string1 = 'char1 char2 char3'
print(len(string1))  # length of a string

chars = list(string1) # returns a list of char
print(chars)

17
['c', 'h', 'a', 'r', '1', ' ', 'c', 'h', 'a', 'r', '2', ' ', 'c', 'h', 'a', 'r', '3']

Changing case¶

# change case
# sets the string to lower case
stringl = string1.lower()
print(stringl)

# sets the string to uper case
stringu = string1.upper()
print(stringu)

stringc = string1.capitalize()
print(stringc)

# note here that syntax is different from list
# output of the method is a new string object since strings are immutable

char1 char2 char3
CHAR1 CHAR2 CHAR3
Char1 char2 char3

Replacement and word splitting¶

string2 = string1.replace('Char2', 'toto')
print(string2)

words = string1.split(' ') #['Char1', 'Char2', 'Char3']
print(words)

words = string1.split(',') #['Char1', 'Char2', 'Char3']
print(words)

char1 char2 char3
['char1', 'char2', 'char3']
['char1 char2 char3']

String formatting¶

sep = ',\n'
# merges a list of strings in one string providing a separator
string3 = sep.join(['toto1', 'toto2','toto3'])
print(string3)

toto1,
toto2,
toto3

# strings behaves  list, no maths with lists! 
# + and * for string concatenations
string4 = 2 * 'toto1' + '\t' + 'toto2' + \
          '\n' + 'toto3 ' + str(10)
print(string4)  

toto1toto1	toto2
toto3 10

# variables to display
x = 10
y = 0.5
z = 0.005

# string formatting. There should be as many %s as variables to display
string5 = '%04d, %.5f, %.3e\n' %(x, y, z) 
# string5 = '%04d, %.5f\n' %(x, y, z) # fails because inconsistent number of var.
print(string5)

0010, 0.50000, 5.000e-03

# If you want to write percentage symbol, use %s (string format)
string5 = 'Percentage:\n%d%s' %(x, '%') 
print(string5)

Percentage:
10%

string5 = ' test string   '
# removing trailing whitespace (usefull when reading a file)
string6 = string5.strip()
print('#' + string5 + '#')
print('#' + string6 + '#')

# test string   #
#test string#

Regular expressions¶

A very powerfull feature is the use of *regular expressions, which allows to match strings with given patterns (re library).

Creating regular expressions¶

# load the regular expression package
import re 

# match string that starts (^) with a number ranging from 0 to 9
pattern1 = r'^[0-9]'
reg1 = re.compile(pattern1)  # creates an object that will be used to match string

# match string that ends ($) with a number ranging from 0 to 9
pattern2 = r'\d$'  # \d is a shortcut for [0-9]
reg2 = re.compile(pattern2)  # creates an object that will be used to match string

Matching regular expressions¶

string1 = r'2-start'
string2 = r'end-3'

reg1.match(string1)  # match: returns a re.Match object

<re.Match object; span=(0, 1), match='2'>

reg2.match(string2)  # no match: returns None

reg2.match(string1)  # returns None (no match)

reg1.match(string2)  # returns a re.Match oject

# note: you should compile a regular expression if will be
# often tested. For isolated cases, you can use:
re.match(pattern1, string1)

<re.Match object; span=(0, 1), match='2'>

Extracting values¶

To extract the values matched by a regular expression, use the groups method. For that, the string pattern that must be extracted must be contained between ().

string1 = r'2-start'
string2 = r'04304-end'
string3 = r'04304-END'
string4 = r'-END'

# Function that returns the groups if a match is not None
def test(match):
    if(match):
        print(match.groups())
    else:
        print('none')

# to get the integer value, use the groups method of the re package
# use () to encompass the elements you want to extract
pattern1 = r'^([0-9]+)-([a-z]+)$'  # + = 1 or more match of the preceding pattern
reg1 = re.compile(pattern1)

test(reg1.match(string1)) # Match 
test(reg1.match(string2)) # Match
test(reg1.match(string3)) # No match (end not of the right case)
test(reg1.match(string4)) # No match (doesnt start with num.)

('2', 'start')
('04304', 'end')
none
none

pattern2 = r'^([0-9]+)-([a-zA-Z]+)$'  # + = 1 or more match of the preceding pattern
reg2 = re.compile(pattern2)

test(reg2.match(string1)) # Match 
test(reg2.match(string2)) # Match
test(reg2.match(string3)) # Match (pattern is now case insensitiv)
test(reg2.match(string4)) # No match (doesnt start with num.)

('2', 'start')
('04304', 'end')
('04304', 'END')
none

pattern3 = r'^([0-9]*)-([a-zA-Z]+)$'  # * = 0 or more match of the preceding pattern
reg3 = re.compile(pattern3)

# All matches
test(reg3.match(string1))
test(reg3.match(string2))
test(reg3.match(string3))
test(reg3.match(string4))

('2', 'start')
('04304', 'end')
('04304', 'END')
('', 'END')

pattern4 = r'^([0-9]?)-([a-zA-Z]+)$'  # ? = 0 or 1 match of the preceding pattern
reg4 = re.compile(pattern4)
test(reg4.match(string1))  # Match
test(reg4.match(string2)) # No match (more that 0 or 1 number at the begining)
test(reg4.match(string3)) # No match (more that 0 or 1 number at the begining)
test(reg4.match(string4))

('2', 'start')
none
none
('', 'END')

Splitting using regular expressions¶

# How to split this string into the three names?    
string1 = r'lala     toto                     titi'
sp1 = string1.split(' ')
print(sp1)
reg = re.compile(' +')  # split based on regular expressions: splits with separator = 1 or more spaces
sp2 = reg.split(string1) 
print(sp2)

['lala', '', '', '', '', 'toto', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', '', 'titi']
['lala', 'toto', 'titi']

string1 = r'01 0304 02 45 509 2950 204' # 302 01 2030 39393 50505 s0304 43df'

# list all the digits of the string
reg = re.compile(r'[0-9]')
print(string1)
print(reg.findall(string1))

01 0304 02 45 509 2950 204
['0', '1', '0', '3', '0', '4', '0', '2', '4', '5', '5', '0', '9', '2', '9', '5', '0', '2', '0', '4']

# List all the 2 to 3 digits. However, 0304 is matched as 030
reg = re.compile(r'[0-9]{2,3}')
print(string1)
print(reg.findall(string1))

01 0304 02 45 509 2950 204
['01', '030', '02', '45', '509', '295', '204']

# better but 0304 is still matched as 304, and 204 is not matched
reg = re.compile(r'[0-9]{2,3} ')  # adding white space at the end
print(string1)
print(reg.findall(string1))

01 0304 02 45 509 2950 204
['01 ', '304 ', '02 ', '45 ', '509 ', '950 ']

# we are close with or statements, but there are white spaces
reg = re.compile(r' [0-9]{2,3} |^[0-9]{2,3} | [0-9]{2,3}$')
print(string1)
print(reg.findall(string1))

01 0304 02 45 509 2950 204
['01 ', ' 02 ', ' 509 ', ' 204']

# Solution: use the \b (word delimiter)
reg = re.compile(r'\b[0-9]{2,3}\b')
print(string1)
print(reg.findall(string1))

01 0304 02 45 509 2950 204
['01', '02', '45', '509', '204']

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