EIW - Perl Regular Expression Exercise Answers

Exercise Answers

Construct regular expressions (match operators) for the following:

any string that contains an "a" or "b" followed by any 2 characters followed by an "a" or a "b". The strings "axxb", "alfa" and "blka" match, and "ab" does not.
[ab] is "either an a or a b".
. is "any character (except newline)".
The entire expression is /[ab]..[ab]/
upper case "A" followed by anything except "x", "y" or "z".
[^xyz] is "anything except an x, y or a z".
The entire expression is /A[^xyz]/
any 5 digit integer.
[0123456789] is "any digit".
[0-9] is another way of saying "any digit".
\d is yet another way of saying "any digit".
The entire expression could be any of the following:
/[0123456789][0123456789][0123456789][0123456789][0123456789]/
/[0-9][0-9][0-9][0-9][0-9]/
/\d\d\d\d\d/
/\d{5}/ (we didn't cover this!)

Develop regular expressions for the following:

Any perl scalar variable name (including the "$"). Perl variable names can contain any alphanumeric character and the "_" character.
```
/\$\w+/
```
Any string that contains nothing but whitespace.
Can't do it without something we haven't covered yet! If you try to use something like /\s+/ it will match any string that contains any whitespace.
An HTML Anchor tag (for example: <A HREF=blahblah>).
```
/<[aA]\s+[hH][rR][eE][fF]=.*>/
```

Develop regular expressions for the following:

Any word (a word is defined as a sequence of alphanumerics - no whitespace) that contains a double letter, for example "book" has a double "o" and "feed" has a double "e".

/([a-zA-Z])\1/
Any string that contains an HTML tag and it's corresponding end tag. The following should match: <H2>Hi Dave</H2> and so should <TITLE>The Test Answers</TITLE>, but this should not match <TITLE>Not a match</H2>. /<(\w+)>.*<\/\1>/
The answer above makes some assumptions about what is inside the angle braces (no whitespace) that are not always true in HTML tags!

Write a perl program that replaces all digits with the name of the digit, so every "0" is replaced with "zero" , "1" is replaced with "one", ... "9" is replaced with "nine".


while (<>) {		# read input one line at a time
	s/0/zero/g;	# replace all "0"s with "zero"
	s/1/one/g;	# replace all "1"s with "one"
	s/2/two/g;	# replace all "2"s with "two"
	s/3/zero/g;	# replace all "3"s with "three"
	s/4/one/g;	# replace all "4"s with "four"
	s/5/two/g;	# replace all "5"s with "five"
	s/6/zero/g;	# replace all "6"s with "six"
	s/7/one/g;	# replace all "7"s with "seven"
	s/8/two/g;	# replace all "8"s with "eight"
	s/9/nine/g;	# replace all "9"s with "nine"
	print;
}

Write a perl program that reads in an HTML file (from STDIN) and replaces all <H1>,</H1> tag pairs with <H3>,</H3> tags.


while (<>) {		    # read input one line at a time
	s/<H1>/<H3>/g;	    # replace all "lt;H1>" with "<H3>"

	s/<\/H1>/<\/H3>/g;  # replace all "</H1>" with "</H3>"
	print;
}

Here is a better way! A single expression that can replace start or end tags.


while (<>) {		    # read input one line at a time
	s/<(\/?)H1>/<\1H3>/g;	# replace all "<H1>" with "<H3>"
				#  "</H1> with "</H3>"
	print;
}

Write a perl program that removes all HTML tags (anything that looks like an HTML tag - you don't need to check each tag name).


while (<>) {		    # read input one line at a time
	s/<[^>]*>//g;	    # remove anything that starts with "<"
			    # and ends with ">"
	print;
}

You might need to think about this one! Write a perl program that strips the HEAD from a HTML file (everything between the <HEAD> tag and the </HEAD> tag. Keep in mind that in HTML newlines mean nothing - any part of a document can be split amongst lines any possible way.

HINT: It is much easier to read the entire sequence of lines in to a single perl scalar variable. Since there are newlines in the single string that contains the entire document - we need to use the "s" modifier to the substitute command if we want "." to match newline.

@lines = <>;            # read everyting until EOF
chop(@lines);	        # get rid if all newlines
$_ = join("",@lines);	# combine lines into one giant string

# remove everything between the first  and the last 
# we need to use the "s" modifier so the ".*" can match a newline!

s/(.*?)<HEAD>.*<\/HEAD>(.*)/\1\2/s;

#print out whatever remains.
print;

Write a perl program that creates a student record in the form used as input to the above program. Each line should contain a student name, followed by a tab (no tabs in the name are allowed), followed by a test1 grade, followed by a tab, etc. A sample output line is:

Joe Smith\t88\t92\t77\n

Your program will accept input in the form of lines that contain name, value pairs with an equal sign (=) between the name and the value. Here is a sample input file:

name = Joe Student
test1 = 86
test2 = 77
homework = 33
name = Jane Smith
test1 = 98
test2 = 35
homework = 85

for this input, the output should be this (\t is a tab):

Joe Student\t86\t77\t33
Jane Smith\t98\t35\t85

Here is one way to do this:

!/usr/bin/perl

# read in all the lines

@lines = <>;

# get rid of all newlines

chomp(@lines);

# remove junk from each line

foreach $i (@lines) {
    $i =~ s/[^=]+=\s(.*)/\1/;
}


#now loop over all lines, handling 4 at a time
for ($i=0;$i<=$#lines;$i=$i+4) {
    print join("\t",@lines[$i..$i+3]), "\n";
}