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.

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


Textbook Time

Check out the following Tutorial:

More Optional Reading


Sed

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


awk

print the first column in the output of a command

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

print first column of values in file separated by :

print second and eighth column separated by ;


awk

print first column of values in a file

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

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

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


Textbook Time

Here is a basic tutorial

Optional Advanced Awk Reading

Last Updated 04/06/2018