There are various different ways you may want to search and replace within a string - and this post shows you how a lot of them work in Python.

1. You may want to replace one literal piece of text by another

2. You may want to replace a string matching a pattern by a literal string

3. You may want to replace a string that matches a pattern by a new string that contains all of (or elements of) the string that matched

4. You may want to replace a string that matches a pattern by a new string that's based on running a piece of code on that matched string.

Replacing one literal string by another

>>> text = "The pea soup club"
>>> text.replace("pea","vegetable")
'The vegetable soup club'
>>>

Replacing a matched string by a literal

This and the following example all use a regular expression:

import re
digits = re.compile(r'(\d+)')

Simply give the replacement string and the incoming string as parameters to the sub method on the regular expression object:

highprice = digits.sub("99",cost)
print highprice

Replacing a matched string by one containing part of the match

* Mark the bit of the incoming string you want to use in the output with capture parentheses

* Use {\1} for the first capture (etc) in the output string in your sub

highprice = digits.sub(r'** {\1} **',cost)
print highprice

Replace a matched string by the result of running code on that match

The example I have used here is an example which takes a numeric vaklue in a string and replaces it by a value that's 5 more; this can't by a simple string replacement ... 17 + 5 => 22 which isn't a straightforward string operations, even in Python.

* Mark the bit of the incoming string you want to calculate on for the output with capture parentheses

* Define a function that converts your captured group(s) into a new string which it returns

* use the "runnable" - i.e. the function - in place of the more usual output string object in your sub

def action(val):
   newval = "%d" % (int(val.group()) + 7)
   return newval
highprice = digits.sub(action,cost)
print highprice

There's a complete source code example from yesterday's Python Programming Course - see [here].
(written 2010-03-25)

 Y108 - Python - String Handling
  [324] The backtick operator in Python and Perl - (2005-05-25)
  [463] Splitting the difference - (2005-10-13)
  [496] Python printf - (2005-11-15)
  [560] The fencepost problem - (2006-01-10)
  [773] Breaking bread - (2006-06-22)
  [903] Pieces of Python - (2006-10-23)
  [943] Matching within multiline strings, and ignoring case in regular expressions - (2006-11-25)
  [954] Splitting Pythons in Bradford - (2006-11-29)
  [970] String duplication - x in Perl, * in Python and Ruby - (2006-12-07)
  [1110] Python - two different splits - (2007-03-15)
  [1195] Regular Express Primer - (2007-05-20)
  [1517] Python - formatting objects - (2008-01-24)
  [1608] Underlining in Perl and Python - the x and * operator in use - (2008-04-12)
  [1876] Python Regular Expressions - (2008-11-08)
  [2284] Strings as collections in Python - (2009-07-12)
  [2406] Pound Sign in Python Program - (2009-09-15)
  [2721] Regular Expressions in Python - (2010-04-14)
  [2765] Running operating system commands from your Python program - (2010-05-14)
  [2780] Formatted Printing in Python - (2010-05-25)
  [2814] Python - splitting and joining strings - (2010-06-16)
  [3090] Matching to a string - what if it matches in many possible ways? - (2010-12-17)
  [3218] Matching a license plate or product code - Regular Expressions - (2011-03-28)
  [3349] Formatting output in Python through str.format - (2011-07-07)
  [3468] Python string formatting - the move from % to str.format - (2011-10-08)
  [3469] Teaching dilemma - old tricks and techniques, or recent enhancements? - (2011-10-08)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3886] Formatting output - why we need to, and first Python example - (2012-10-09)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4152] Why are bus fares so high? - (2013-08-18)
  [4213] Formatting options in Python - (2013-11-16)
  [4307] Identifying and clearing denial of service attacks on your Apache server - (2014-09-27)
  [4360] Python - comparison of old and new string formatters - (2014-12-22)
  [4593] Command line parameter handling in Python via the argparse module - (2015-12-08)
  [4595] Python formatting update - including named completions - (2015-12-10)
  [4659] Prining a pound sign from Python AND running from the command line at the same time - (2016-03-03)

Back to New brochures for the Melksham area

Previous and next or Horse's mouth home

Forward to Methods that run on classes (static methods) in Python

Garlic bread without garlic
TCP v UDP / Client v Server - Python examples
Multiple processes (forking) in Python
Methods that run on classes (static methods) in Python
Flexible search and replace in Python
New brochures for the Melksham area
The World Company Register - is it another scam?
Can my dog eat potatoes? Doggie Dietary Research, and political sleaze!
Security considerations in programming - what do we teach?
A lovely spring afternoon