-Joseph Beberman
*Some slides are inspired by a PowerPoint
presentation used by professor Seikyung Jung,
which was derived from Charlie Wiseman.
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Regular expressions are all over the place.
All syntaxes are almost identical, but for what
it’s worth I will be using the syntax tied to
Unix systems.
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In computer science, regular expressions are used to
locate strings based on a pattern.
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Search for every email address in a file? Regular
expressions make it easy.
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Often referred to as regexp, regex, etc.
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For instance: A phone number is three digits, followed by a
dash, three digits, a dash, and then four digits (“555-5555555”). You can make a regular expression which matches
all phone numbers by indicating that pattern.
For reference: [0-9]{3}-[0-9]{3}-[0-9]{4}
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Consider values of currency. How could you
describe in English any/all monetary values
(including cents) with a single pattern?
ie. $50.25
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Consider values of currency. How could you
describe in English any/all monetary values
(including cents) with a single pattern?
ie. $50.25
Dollar sign, any number of digits, period, two
digits.
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Consider values of currency. How could you
describe in English any/all monetary values
(including cents) with a single pattern?
ie. $50.25
Dollar sign, any number of digits, period, two
digits.
For reference: \$[0-9]+\.[0-9]{2}
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Using it to showcase regular expressions.
◦ It actually stands for Global Regular Expression
Parser
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A command available on most if not all Unixlike systems.
Seem to be incredibly popular command for
system administrators.
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grep is used to do text-based searching,
generally on the Linux command line or in
scripting
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Takes two arguments
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Generic format: grep STRING FILE
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It prints every line of FILE that has STRING in it.
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Example: grep root /etc/passwd
◦ »Prints out all lines in the /etc/passwd file that contain
the string "root"
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The contents of /etc/passwd
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grep ‘root’ /etc/passwd
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What does this tell you?
◦ 2 lines contain the string “root”
◦ Highlights exactly where the string was matched
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Grep has a number of options, and even
though it’s off topic knowing some may help
you understand the power of grep/regexps.
-i » ignore case
-v » negation
◦ grep –v hello filename.txt
 Would return every line of filename.txt without the
word hello in it.
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How does grep use regular expressions?
◦ Again: stands for Global Regular Expression Parser
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Recall the format: grep STRING FILE
The STRING is actually interpreted as a
regular expression.
Note: I will be using the –E option for grep
◦ Don’t worry about it, it essentially enables all
regexp functionality.
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First thing’s first… we need a text file to
search!
I’ve taken the time to make a simple text file
which will help me show some simpler
regular expressions.
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How do you up your game from literal strings like
“root”, to creating patterns? Regexs have their
own syntax.
To start: parenthesis are used for grouping “or
statements”.
To match one thing or something else, you group
them in parenthesis and separate them with
pipes.
◦ (joe|Joe) will match the string “joe” or “Joe”
◦ (hello|goodbye|sup) matches “”hello” “goodbye” or “sup”
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You can specify a range of characters within
brackets.
◦ For example [a-z] will match any lower case letter.
◦ [A-Z] any upper case letter
◦ [0-9] any digit
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Now the pattern is any digit.
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Now the pattern is digits 0 to 5.
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You can match one thing after another.
◦ For example: [a-z][0-9] will match any lower case
letter followed by a number. Now we are starting to
see patterns!
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When specifying one range or another, you
don’t need a pipe.
◦ For example [a-zA-Z] will match any lower or upper
case letter.
◦ [0-9a-zA-Z] will match any alphanumeric character
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Now it’s time to get more specific. What if
you want to find something that occurs
multiple times in a row?
The +, *, ?, and {} special characters specify
how many times you want the pattern directly
in front of them to occur.
◦ Ex. [a-zA-Z]+
◦ The + modifies the grouping in front of it
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+ » one or more instances
◦ [a-zA-Z]+ would match any string of lower/upper case
letters at least 1 letter long.
* » zero or more instances
◦ [0-9]* would match any number of digits, or none at all.
? » zero or one instance (aka optional)
◦ [a-zA-Z]+ would match a single letter or none at all.
[a-z]+[0-9]*[A-Z]?
◦ ade7E
◦ cpB
◦ F12CP X
◦ Please ask questions here if you’re confused!
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{} » specific or range
◦ {3} or {4,7}
◦ ‘[0-9]{3}-[0-9]{3}-[0-9]{4}’ for a phone number
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Now we can make a regular expression that matches
emails!
Let’s try now…
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Now we can make a regular expression that matches
emails!
Let’s try now…
Any alphanumeric sequence, @, any
alphabetical sequence, ., any lower case
sequence
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Now we can make a regular expression that matches
emails!
Let’s try now…
Any alphanumeric sequence, @, any
alphabetical sequence, ., any lower case
sequence
‘[a-zA-Z0-9]+’
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Now we can make a regular expression that matches
emails!
Let’s try now…
Any alphanumeric sequence, @, any
alphabetical sequence, ., any lower case
sequence
‘[a-zA-Z0-9]+@’
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Now we can make a regular expression that matches
emails!
Let’s try now…
Any alphanumeric sequence, @, any
alphabetical sequence, ., any lower case
sequence
‘[a-zA-Z0-9]+@[a-zA-Z]+’
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Now we can make a regular expression that matches
emails!
Let’s try now…
Any alphanumeric sequence, @, any
alphabetical sequence, ., any lower case
sequence
‘[a-zA-Z0-9]+@[a-zA-Z]+.’
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Now we can make a regular expression that matches
emails!
Let’s try now…
Any alphanumeric sequence, @, any
alphabetical sequence, ., any lower case
sequence
‘[a-zA-Z0-9]+@[a-zA-Z]+.[a-z]+’
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Weird… why did we match that third line?
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. Is a special character which takes the place of
anything.
That means ‘t.o’ would match two, too, t2o, or
many other things.
That’s how it matched below. The . matched 0!
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So how do we avoid matching weird things like
j03b130@h0tma?
◦ [a-zA-Z0-9]+@[a-zA-Z]+\.[a-z]+’
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You can escape special characters by putting \ in
front of them.
◦ So \. means a literal period.
◦ Note: Escape \ by putting \ in front of it! \\
 So \\ means a literal back slash.
◦ Double Note: the space character is matched by \s
 effectively escaping the s character.
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^ » Indicates the start of a line
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Notice how it didn’t match ever line with “I” in
it, only the ones which start with I.
Vs.
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$ » indicates end of a line
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Syntax:
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^ start line
$ end line
+ one or more
* zero or more
? zero or one
. replace with anything
{n} n times
{n, m} n to m times
(string1|string2) matches string1 or string2
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What does this match?
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[0-9]{3}-[0-9]{3}-[0-9]{4}
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What does this match?
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[0-9]{3}-[0-9]{3}-[0-9]{4}
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Phone numbers!
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What does this match?
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\$[0-9]+\.[0-9]{2}
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Money values
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Example: What does this match?
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‘(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)'
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Example: What does this match?
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‘(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?\.){3}(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)‘
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That actually matches valid IP addresses.
(I found it online though. Credit to SASIKALA of thegeekstuff.com)
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Regular expressions simply indicate a pattern.
What is important is that the pattern can be
searched for as opposed to a literal string.
That means instead of searching for a specific
phone number string input, you can search for
any existing phone number with ease by
matching the pattern that all phone numbers
follow.
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Common tasks that regular expressions are used for:
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It finds strings that match a given syntax.
◦ -Ctrl-F, anyone? There are tools to add regular expression
functionality to Ctrl-F, at least on Chrome.
◦ -Tool: Regular Expression Searcher
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Once you find said strings based on the pattern, there
are limitless possibilities as to what you can do with
those matches.
Substitution: Replace all matching strings.
◦ -Ctrl-H (on word), anyone?
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Splitting: Split strings based upon matches.