Search and Replace

As well as being excellent for search, Python is also great at doing search and replace. Create a new Python script (replace.py) and copy the following;

import sys
import re

lines = open( sys.argv[1], "r" ).readlines()

for line in lines:
    line = re.sub( r"the", "THE", line )
    print line,

This script reads in a file and prints out every line to the screen. However, before printing the line, it modifies it using the sub (substitute) function, re.sub( r"the", "THE", line ). This sub function is a lot like the search function, except that now there is an extra argument (THE) that is the replacement string. This searches for the text in the first string r"the" and replaces it with the text in the second string "THE" (so in this case this replaces the with THE). Note that this replaces all occurances of the with THE. You can optionally specify the maximum number of matches, e.g.

import re

line = "Round the ragged rock the ragged rascal ran"

# replace only a maximum of 2 matches of "ra" with "RA"
line = re.sub( r"ra", "RA", line, 2 )

print line

The sub function performs a case-sensitive substitution. Case-insensitive substitution is a little more complex... For example;

import re

#replace all occurances of "the", "The", "THe" etc. with "THE"
line = re.sub( re.compile( r"the", re.IGNORECASE ), "THE", line )

In this last case, we've had to use re.compile to compile a search expression that could perform a case-insensitive search for the.

Sometimes you may want to perform the substitution at a specific place in the line, e.g. only at the beginning of the line or only at the end. You can do this by adding either a carat (^) to the beginning of the search string, to force matching at the beginning, or by adding a dollar sign ($) to the end of the search string to force matching at the end, e.g.;

import re

line = "is this the bliss that this is"

# replace all instances of "is" with "IS"
print re.sub( r"is", "IS", line )

# replace only the first two instances of "is" with "IS"
print re.sub( r"is", "IS", line, 2 )

# replace only an "is" at the beginning of the string
print re.sub( r"^is", "IS", line )

# replace only the "is" at the end of the string
print re.sub( r"is$", "IS", line )

You can also use variables in the search and replace parts of the substitute string, e.g.

import re

search = "the"
replace = "THE"

#case-insensitive replace "the" with "THE"
line = re.sub( re.compile(search, re.IGNORECASE), replace, line )

Exercise

Use search and replace to update your grep.py script so that it not only prints matching lines, but it also highlights the matched string in the line (e.g. by adding asterisks around the word, or by capitalising the word).

(note, you can capitalise a string by writing line = line.upper(). Similarly, you can lower-case a string by writing line = line.lower())

Here a possible answer.