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CSE 391

Regular Expressions�grep

Slides created by Josh Ervin and Hunter Schafer. �Based off slides made by Marty Stepp, Jessica Miller, Ruth Anderson, Brett Wortzman, and Zorah Fung

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ROADMAP

Introduction to the command line
Input/output redirection, pipes
More input/output redirection, tee, xargs
Git: Fundamentals
Git: Branches, merging, and remote repositories
Regular expressions
More regular expressions, sed
Users and permissions
Bash scripting
Industry applications

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AGENDA

Regular expressions

Regular expression syntax
Get started early-ish on the next homework assignment!

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REGULAR EXPRESSIONS

A regular expression, often referred to as a regex, is a description of a pattern of text.

Have a formal mathematical definition (DFA’s in CSE 311)

There are countless applications of regular expressions

Search for text in a given file (Purpose of this lecture)
Search/Replace text in a file

Virtually all programming languages implement regular expressions

You’ve all see this before in java using String split (Grammar Solver, split on whitespace)
Regular expressions differ between programming languages
We will be using regular expressions in the context of grep.

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SIMPLE REGEX

grep -E “Chocolate” candies.txt

Notes:

grep is case-sensitive
Notice that running grep from a terminal prints out the entire line

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -Ei “Chocolate” candies.txt

Notes:

We can make grep case-insensitive with -i
Notice that running grep from a terminal prints out the entire line

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “a” candies.txt

Notes:

grep is case-sensitive
grep will highlight multiple matches per line

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -Ei “a” candies.txt

Notes:

We can make grep case-insensitive with -i
grep will highlight multiple matches per line

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “ar” candies.txt

Notes:

When we type a literal string (i.e. “ar”), grep will only print exact matches to this string. Subsets are not matches.

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “.a” candies.txt

Notes:

The . character matches anything

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “^K” candies.txt

Notes:

^ matches the beginning of the line

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “\<T” candies.txt

Notes:

\< matches the start of a word
A word is considered a string of characters consisting of only letters, numbers, and underscores

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “t\>” candies.txt

Notes:

\> matches the end of a word
A word is considered a string of characters consisting of only letters, numbers, and underscores

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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SIMPLE REGEX

grep -E “\.” candies.txt

Notes:

\ is an escape character.
For the example above, this means “search for a literal period”

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat.

Dark chocolate

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GLOSSARY

Syntax	Functionality
.	Any character
^	Start of line
$	End of line
\<	Start of word
\>	End of word
\	Escape the following character
-i	(Flag to grep) match case insensitively

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Syntax	Functionality
.	Any character
^	Start of line
$	End of line
\<	Start of word
\>	End of word

candies.txt

Twix

Sweet Tart

Chocolate

Almond Joy

Jolly Ranchers

Kit Kats

Dark chocolate

Suppose we have the file candies.txt on the left. What is the full grep command to print out all lines that contain four-letter words that start with the letter T (Uppercase)?

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REGEX

grep -E “Twix|Tarts” candies.txt

Notes:

| is the logical or, it matches the thing on the left or the thing on the right.
WARNING: be really careful of spaces. Leading/trailing spaces after the | are interpreted literally. For example: “Twix | Tarts” searches for “Twix “ and “ Tarts”

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat

Dark chocolate

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REGEX

grep -E “(e|a)t” candies.txt

Notes:

You can use parentheses to capture groups and encapsulate logical operators.
The above regex looks for all strings of two characters: either et or at

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat

Dark chocolate

Reeses

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REGEX

grep -E “e*t” candies.txt

Notes:

The * operator means “zero or more of the previous pattern”
This translates to “zero or more e’s followed by a t”

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat

Dark chocolate

Reeses

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REGEX

grep -E “e+t” candies.txt

Notes:

The + operator means “one or more of the previous pattern”
This translates to “one or more e’s followed by a t”

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat

Dark chocolate

Reeses

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REGEX

grep -E “r?t” candies.txt

Notes:

The ? operator means “zero or one of the previous pattern”
This translates to “rt or t”

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat

Dark chocolate

Reeses

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REGEX

grep -E “(es)+” candies.txt

Notes:

Wildcard operators, such as *, +, and ? can be applied to groups of characters using parentheses

candies.txt

Twix

Sweet Tarts

Chocolate

Almond Joy

Jolly Ranchers

Kit Kat

Dark chocolate

Reeses

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GLOSSARY

Syntax	Functionality
\|	Logical or
*	Zero or more of
+	One or more of
?	Zero or one of
()	Group characters together

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Syntax	Functionality
\|	Logical or
*	Zero or more
+	One or more
?	Zero or one
()	Group characters

kitkats.txt

KitKats

Kit Kats

Kit kat

kitkats

Kit Kats

kitkat

kitkats

KITKATS

Suppose we have the file candies.txt on the left. We want to print out all lines containing kitkats. All of the following criteria are valid:

Starts with an upper or lowercase K
May or may not contain a space after the first t
The second K may be upper or lowercase
Optionally ends with an s

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REGEX

grep -E “[abc]” passwords.txt

Notes:

[ ] denotes a character set. It is equivalent to an or statement, but is easier to read

[abc] = (a|b|c)

passwords.txt

password123

supersecret

NoHackingPlz

122password

Not a valid password.

won’t remember

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REGEX

grep -E “[a-z]” passwords.txt

Notes:

a-z is a range of characters. It’s some shorthand included to make things like letters and numbers easier to write

a-z is all lowercase letters
A-Z is all uppercase letters
0-9 is all digits
a-zA-Z is all upper and lowercase letters

passwords.txt

password123

supersecret

NoHackingPlz

122password

Not a valid password.

won’t remember

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REGEX

grep -E “[.]” passwords.txt

Notes:

Special characters inside of character sets do not usually need to be escaped.

passwords.txt

password123

supersecret

NoHackingPlz

122password

Not a valid password.

won’t remember

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REGEX

grep -E “[^ao]” passwords.txt

Notes:

When inside of a character set, ^ negates all of the contents of it.
WARNING: This is different from the ^ outside of the character set, which denotes the start of the line.

passwords.txt

password123

supersecret

NoHackingPlz

122password

Not a valid password.

won’t remember

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REGEX

grep -E “[0-9]{2}” passwords.txt

Notes:

Curly braces are another type of modifier that allow us to specify how many of a certain match we want
{x} - match exactly x characters of the pattern
{x,} - match x or more characters of the pattern
{,x} - match x or fewer characters of the pattern
{x,y} - match between x and y characters of the pattern
All of these ranges are inclusive

passwords.txt

password123

supersecret

NoHackingPlz

122password

Not a valid password.

won’t remember

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REGEX

grep -E “(..)\1” passwords.txt

Notes:

The \1 is a back reference - it is used to reference an earlier match.
The above string is search for “any two characters, followed immediately by those same two characters.”
If you have more than one grouping (i.e. things captured by parentheses) you can reference them with \1, \2, \3, etc

passwords.txt

password123

supersecret

NoHackingPlz

122password

Not a valid password.

won’t remember

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GLOSSARY

Syntax	Functionality
[ ]	Character set
[^ ]	Negate character set
[a-z]	All lowercase characters
[A-Z]	All uppercase characters
[0-9]	All digits
\1	Back reference earlier character