Search and Replace
As well as being excellent for search, Python is also great at doing search and replace. Create a new Python script (
nano replace.py) and copy the following;
from __future__ import print_function import sys import re lines = open( sys.argv, "r" ).readlines() for line in lines: line = re.sub( r"the", "THE", line ) print(line, end="")
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). Note that this replaces all occurances of
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 line = "The THEsis is the thEory of Theocracy" #replace all occurances of "the", "The", "THe" etc. with "THE" line = re.sub( re.compile( r"the", re.IGNORECASE ), "THE", line ) print(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" line = "The THEsis is the thEory of Theocracy" #case-insensitive replace "the" with "THE" line = re.sub( re.compile(search, re.IGNORECASE), replace, line ) print(line)
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.