Regular Expressions

Is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager.

Essentially allows us to find a pattern in a string.

Regexes are widely supported in many programming languages including Java, Python, C++, Perl, Javascript, and PHP. It is also supported in text processing programs advanced text editors, and some other programs.

Regular Expressions

Here are SOME of the main rules:

     Text:
      .           Any single character
      [chars]     Character class: Any character of the class ``chars''
      [^chars]    Character class: Not a character of the class ``chars''
      text1|text2 Alternative: text1 or text2

     Quantifiers:
      ?           0 or 1 occurrences of the preceding text
      *           0 or N occurrences of the preceding text (N > 0)
      +           1 or N occurrences of the preceding text (N > 1)

     Grouping:
      (text)      Grouping of text (used either to set the borders of an alternative as above, or to make backreferences, where the Nth group can be referred to on the RHS of a RewriteRule as $N)

     Anchors:
      ^           Start-of-line anchor
      $           End-of-line anchor

Regular Expression examples

I will use grep to demonstrate, but there are other programs that could be used.

  • egrep aa /usr/share/dict/words #find all words with an 'aa' in it.

The basic syntax is grep regex file. Below I will just write the regex.

  • ^aa words that start with an 'aa'
  • aa$ words that end with 'aa'

Textbook Time

Check out the following Tutorial:

More Optional Reading

Sed

  • Stands for 'stream editor' which allows us to filter and transform text.
  • Often used in conjunction with regex

Substitution

Replaces all instances of day with night inside myfile.txt. Note the g at the end.

sed 's/day/night/g' myfile.txt

Sed

Removing stuff

Do not print the first line

sed '1d' file.txt

Remove the first character of every line

sed 's/^.//' file

Sed

Remove the last character of every line

sed 's/.$//' file

Remove lines 7 thru 9 of a file

sed '7,9d' filename.txt

Remove the line containing the string Fred

sed '/Fred/d' filename.txt

Sed

Print every nth line beginning with in the file

Print only the first line

sed -n '1p' file.txt

Print every third line starting from line 3 (which will print lines 3, 6, 9, etc)

sed -n '0~3p' file.txt

Sed

Print every fifth line starting with the second line (which will print lines 2, 7, 12, 17, etc)

sed -n '2~5p' file.txt

Print a range of lines

Print lines 1 through 4

sed -n '1,4p' file.txt

Print lines 2 through 4

sed -n '2,4p' file.txt

Textbook Time

Read the following webpage:

Optional Reading

awk

Awk is hugely powerful, but we will just look at how it can be used for text pattern scanning.

It is also often used with Regular Expressions.

Here are some examples:

print the first and third columns in the output of a command

  • ls -l | awk '{ print $1 $3 }'

awk

print the first column in the output of a command

  • ps aux | awk '{ print $1 }'

print the first column in the output of a command and add text.

  • ps aux | awk '{ print "this process is owned by " $1 }'

print first column of values in file separated by :

  • awk -F: '{ print $1 }' /etc/passwd

print second and eighth column separated by ;

  • awk -F';' '{ print $2, $8 }' master_file_room_104.dhcp

awk

print first column of values in a file

  • awk '{ print $1 }' /etc/fstab

Using regular expressions search for lines that start with UUID and print 3rd column of results

  • awk '/^UUID/ {print $3}' /etc/fstab

Figure this one out - if you don't know what the lspci -v command does - read the man page.

  • lspci -v | awk '/VGA/ { print $6 }'

Awk does Arithmetic operations too (assuming grades is a file with 3 scores on each row)

  • awk '{ print "the average is ", ($1+$2+$3)/3 }' grades

Textbook Time

Here is a basic tutorial

Optional Advanced Awk Reading