Regular Expressions

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Software Tools

• Regular Expressions

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What is a Regular Expression?

• A regular expression is a pattern to be matched
  against a string. For example, the pattern Bill.
• Matching either succeeds or fails.
• Sometimes you may want to replace a matched
  pattern with another string.
• Regular expressions are used by many other Unix
  commands and programs, such as grep, sed, awk,
  vi, emacs, and even some shells.

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Simple Uses of Regular Expressions

• If we are looking for all the lines in a file
  that contain the string Shakespeare, we could use
  the grep command
• grep Shakespeare movie result
• Here, Shakespeare is the regular expression that
  grep looks for in the file movie.
• Lines that match are redirected to result.

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Simple Uses of Regular Expressions

• In Perl, we can make Shakespeare a regular
  expression by enclosing it in slashes
• if(/Shakespeare/)
• print _
• What is tested in the if-statement?
• Answer _.
• When a regular expression is enclosed in
  slashes, _ is tested against the regular
  expression, returning true if there is a match,
  false otherwise.

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Simple Uses of Regular Expressions

• if(/Shakespeare/)
• print _
• The previous example tests only one line, and
  prints out the line if it contains Shakespeare.
• To work on all lines, add a loop
• while()
• if(/Shakespeare/)
• print

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Simple Uses of Regular Expressions

• What if we are not sure how to spell Shakespeare?
• Certainly the first part is easy Shak, and there
  must be a r near the end.
• How can we express our idea?
• grep grep "Shak.r" movie result
• Perl while()
• if(/Shak.r/)
• print
• . means zero or more of any character.

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Simple Uses of Regular Expressions

• grep grep "Shak.r" movie result
• The double quotes in this grep example are needed
  to prevent the shell from interpreting as all
  files.
• Since Shakespeare ends in e, shouldnt it be
• Shak.r.
• Answer No need. Any character can come before
  or after the pattern.
• Shak.r is the same as .Shak.r.

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Substitution

• Another simple regular expression is the
  substitute operator.
• It replaces part of a string that matches the
  regular expression with another string.
• s/Shakespeare/Bill Gates/
• _ is matched against the regular expression
  (Shakespeare).
• If the match is successful, the part of the
  string that matched is discarded and replaced by
  the replacement string (Bill Gates).
• If the match is unsuccessful, nothing happens.

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Substitution

• The program
• cat movie
• Titanic
• Saving Private Ryan
• Shakespeare in Love
• Life is Beautiful
• cat sub1
• !/usr/local/bin/perl5 -w
• while()
• if(/Shakespeare/)
• s/Shakespeare/Bill Gates/
• print
• sub1 movie
• Bill Gates in Love

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Substitution

• An even shorter way to write it
• cat sub2
• !/usr/local/bin/perl5 -w
• while()
• if(s/Shakespeare/Bill Gates/)
• print
• sub2 movie
• Bill Gates in Love

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Patterns

• A regular expression is a pattern.
• Some parts of the pattern match a single
  character (a).
• Other parts of the pattern match multiple
  characters (.).

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Single-Character Patterns

• The dot . matches any single character except
  the newline (\n).
• For example, the pattern /a./ matches any
  two-letter sequence that starts with a and is not
  a\n.
• Use \. if you really want to match the period.
• cat test
• hi
• hi bob.
• cat sub3
• !/usr/local/bin/perl5 -w
• while()
• if(/\./) print
• sub3 test
• hi bob.

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Single-Character Groups

• If you want to specify one out of a group of
  characters to match use
• /abcde/
• This matches a string containing any one of the
  first 5 lowercase letters, while
• /aeiouAEIOU/
• matches any of the 5 vowels in either upper or
  lower case.

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Single-Character Groups

• If you want in the group, put a backslash
  before it, or put it as the first character in
  the list
• /abcde/ matches abcde
• /abcde\/ okay
• /abcde/ also okay
• Use - for ranges of characters (like a through
  z)
• /0123456789/ any single digit /0-9/
  same
• If you want - in the list, put a backslash before
  it, or put it at the beginning/end
• /X-Z/ matches X, Y, Z
• /X\-Z/ matches X, -, Z
• /XZ-/ matches X, Z, -
• /-XZ/ matches -, X, Z

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Single-Character Groups

• More range examples
• /0-9\-/ match 0-9, or minus
• /0-9a-z/ match any digit or lowercase
  letter
• /a-zA-Z0-9_/ match any letter, digit,
  underscore
• There is also a negated character group, which
  starts with a immediately after the left
  bracket. This matches any single character not in
  the list.
• /0123456789/ match any single
  non-digit /0-9/ same
• /aeiouAEIOU/ match any single non-vowel
• /\/ match any single character except

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Single-Character Groups

• For convenience, some common character groups are
  predefined
• Predefined Group Negated Negated Group
• \d (a digit) 0-9 \D (non-digit) 0-9
• \w (word char) a-zA-Z0-9_ \W
  (non-word) a-zA-Z0-9_
• \s (space char) \t\n \S (non-space) \t\n
• \d matches any digit
• \w matches any letter, digit, underscore
• \s matches any space, tab, newline
• You can use these predefined groups in other
  groups
• /\da-fA-F/ match any hexadecimal digit

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Multipliers

• Multipliers allows you to say one or more of
  these or up to four of these.
• means zero or more of the immediately previous
  character (or character group).
• means one or more of the immediately previous
  character (or character group).
• ? means zero or one of the immediately previous
  character (or character group).

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Multipliers

• Example
• /Gate?s/
• matches a G followed by one or more as followed
  by t, followed by an optional e, followed by s.
• , , and ? are greedy, and will match as many
  characters as possible _ "Bill xxxxxxxxx
  Gates"
• s/x/Cheap/ gives Bill Cheap Gates

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General Multiplier

• How do you say five to ten xs?
• /xxxxxx?x?x?x?x?/ works, but ugly
• /x5,10/ nicer
• How do you say five or more xs?
• /x5,/
• How do you say exactly five xs?
• /x5/
• How do you say up to five xs?
• /x0,5/

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General Multiplier

• How do you say c followed by any 5 characters
  (which can be different) and ending with d?
• /c.5d/
• is the same as 0,
• is the same as 1,
• ? is the same as 0,1

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Pattern Memory

• How would we match a pattern that starts and ends
  with the same letter or word?
• For this, we need to remember the pattern.
• Use ( ) around any pattern to put that part of
  the string into memory (it has no effect on the
  pattern itself).
• To recall memory, include a backslash followed by
  an integer.
• /Bill(.)Gates\1/

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Pattern Memory

• Example
• /Bill(.)Gates\1/
• This example matches a string starting with
  Bill, followed by any single non-newline
  character, followed by Gates, followed by that
  same single character.
• So, it matches
• Bill!Gates! Bill-Gates-
• but not
• Bill?Gates! Bill-Gates_
• (Note that /Bill.Gates./ would match all four)

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Pattern Memory

• More examples
• /a(.)b(.)c\2d\1/
• This example matches a string starting with a, a
  character (1), followed by b, another single
  character (2), c, the character 2, d, and the
  character 1.
• So it matches a-b!c!d-.

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Pattern Memory

• The reference part can have more than a single
  character.
• For example
• /a(.)b\1c/
• This example matches an a, followed by any number
  of characters (even zero), followed by b,
  followed by the same sequence of characters,
  followed by c.
• So it matches aBillbBillc and abc, but not
  aBillbBillGatesc.

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Alteration

• How about picking from a set of alternatives when
  there is more than one character in the patterns.
  
• The following example matches either Gates or
  Clinton or Shakespeare
• /GatesClintonShakespeare/
• For single character alternatives,
• /abc/
• is the same as
• /abc/.

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Anchoring Patterns

• Anchors requires that the pattern be at the
  beginning or end of the line.
• matches the beginning of the line (only if is
  the first character of the pattern)
• /Bill/ match lines that begin with Bill
• /Gates/ match lines that begin with Gates
• /Bill\/ match lines containing Bill
  somewhere
• /\/ match lines containing
• matches the end of the line (only if is the
  last character of the pattern)
• /Bill/ match lines that end with Bill
• /Gates/ match lines that end with Gates
• /Bill/ match with contents of scalar Bill
• /\/ match lines containing

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Precedence

• So what happens with the pattern ab
• Is this (ab) or
• a(b) ?
• Precedence of patterns from highest to lowest
• Name Representation
• Parentheses ( )
• Multipliers ? m,n
• Sequence anchoring abc
• Alternation
• By the table, has higher precedence than , so
  it is interpreted as a(b).

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Precedence

• What if we want the other interpretation in the
  previous example?
• Answer Simple, just use parentheses (ab)
• Use parentheses in ambiguous cases to improve
  clarity, even if not strictly needed.
• When you use parentheses for precedence, they
  also go into memory (\1, \2, \3).

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Precedence

• More precedence examples
• abc matches ab, abc, abcc, abccc,
• (abc) matches "", abc, abcabc, abcabcabc,
• ab matches a at beginning of line, or b
  anywhere
• (ab) matches either a or b at the beginning
  of line
• abcd a, or bc, or d
• (ab)(cd) ac, ad, bc, or bd
• (Bill Gates)(Bill Clinton) Bill Gates, Bill
  Clinton
• Bill (GatesClinton) Bill Gates, Bill Clinton
• (Mr\. Bill)(Bill (GatesClinton))
• Mr. Bill, Bill Gates, Bill Clinton
• (Mr\. )?Bill( Gates Clinton)?
• Bill, Mr. Bill, Bill Gates, Bill Clinton,
• Mr. Bill Gates, Mr. Bill Clinton

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• What if you want to match a different variable
  than _?
• Answer Use .
• Examples
• name "Bill Shakespeare"
• name /Bill/ true
• name /(.)\1/ also true (matches ll)
• if(name /(.)\1/)
• print "name\n"

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• An example using to match
• cat match1
• !/usr/local/bin/perl5 -w
• print "Quit (y/n)? "
• if( /yY/)
• print "Quitting\n"
• exit
• print "Continuing\n"
• match1
• Quit (y/n)? y
• Quitting

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• Another example using to match
• cat match2
• !/usr/local/bin/perl5 -w
• print "Wakeup (y/n)? "
• while( /nN/)
• print "Sleeping\n"
• print "Wakeup (y/n)? "
• match2
• Wakeup (y/n)? n
• Sleeping
• Wakeup (y/n)? N
• Sleeping
• Wakeup (y/n)? y

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Ignoring Case

• In the previous examples, we used yY and nN
  to match either upper or lower case.
• Perl has an ignore case option for pattern
  matching /somepattern/i
• cat match1a
• !/usr/local/bin/perl5 -w
• print "Quit (y/n)? "
• if( /y/i)
• print "Quitting\n"
• exit
• print "Continuing\n"
• match1a
• Quit (y/n)? Y
• Quitting

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Slash and Backslash

• If your pattern has a slash character (/), you
  must precede each with a backslash (\)
• cat slash1
• !/usr/local/bin/perl5 -w
• print "Enter path "
• path
• if(path /\/usr\/local\/bin/)
• print "Path is /usr/local/bin\n"
• slash1
• Enter path /usr/local/bin
• Path is /usr/local/bin

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Different Pattern Delimiters

• If your pattern has lots of slash characters (/),
  you can also use a different pattern delimiter
  with the form msomepattern
• The can be any non-alphanumeric character.
• cat slash1a
• !/usr/local/bin/perl5 -w
• print "Enter path "
• path
• if(path m/usr/local/bin)
• if(path m_at_/usr/local/bin_at_) also works
• print "Path is /usr/local/bin\n"
• slash1a
• Enter path /usr/local/bin
• Path is /usr/local/bin

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Special Read-Only Variables

• After a successful pattern match, the variables
  1, 2, 3, are set to the same values as \1,
  \2, \3,
• You can use 1, 2, 3, later in your program.
• cat read1
• !/usr/local/bin/perl5 -w
• _ "Bill Shakespeare in Love"
• /(\w)\W(\w)/ match first two words
• 1 is now "Bill" and 2 is now "Shakespeare"
• print "The first name of 2 is 1\n"
• read1
• The first name of Shakespeare is Bill

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Special Read-Only Variables

• You can also use 1, 2, 3, by placing the
  match in a list context
• cat read2
• !/usr/local/bin/perl5 -w
• _ "Bill Shakespeare in Love"
• (first, last) /(\w)\W(\w)/
• print "The first name of last is first\n"
• read2
• The first name of Shakespeare is Bill

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Special Read-Only Variables

• Other read-only variables
• is the part of the string that matched the
  pattern.
• is the part of the string before the match
• is the part of the string after the match
• cat read3
• !/usr/local/bin/perl5 -w
• _ "Bill Shakespeare in Love"
• / in /
• print "Before \n"
• print "Match \n"
• print "After '\n"
• read3
• Before Bill Shakespeare
• Match in
• After Love

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More on Substitution

• If you want to replace all matches instead of
  just the first match, use the g option for
  substitution
• cat sub3
• !/usr/local/bin/perl5 -w
• _ "Bill Shakespeare in love with Bill
  Gates"
• s/Bill/William/
• print "Sub1 _\n"
• _ "Bill Shakespeare in love with Bill
  Gates"
• s/Bill/William/g
• print "Sub2 _\n"
• sub3
• Sub1 William Shakespeare in love with Bill
  Gates
• Sub2 William Shakespeare in love with William
  Gates

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More on Substitution

• You can use variable interpolation in
  substitutions
• cat sub4
• !/usr/local/bin/perl5 -w
• find "Bill"
• replace "William"
• _ "Bill Shakespeare in love with Bill
  Gates"
• s/find/replace/g
• print "_\n"
• sub4
• William Shakespeare in love with William Gates

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More on Substitution

• Pattern characters in the regular expression
  allows patterns to be matched, not just fixed
  characters
• cat sub5
• !/usr/local/bin/perl5 -w
• _ "Bill Shakespeare in love with Bill Gates"
• s/(\w)//g
• print "_\n"
• sub5
• 
  

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More on Substitution

• Substitution also allows you to
• ignore case
• use alternate delimiters
• use
• cat sub6
• !/usr/local/bin/perl5 -w
• line "Bill Shakespeare in love with bill
  Gates"
• line sbillWilliamgi
• line s_at_Shakespeare_at_Gates_at_gi
• print "line\n"
• sub6
• William Gates in love with William Gates

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split

• The split function allows you to break a string
  into fields.
• split takes a regular expression and a string,
  and breaks up the line wherever the pattern
  occurs.
• cat split1
• !/usr/local/bin/perl5 -w
• line "Bill Shakespeare in love with Bill
  Gates"
• _at_fields split(/ /,line)
• split line using space as delimiter
• print "fields0 fields3 fields6\n"
• split1
• Bill love Gates

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split

• You can use _ with split.
• split defaults to look for space delimiters.
• cat split2
• !/usr/local/bin/perl5 -w
• _ "Bill Shakespeare in love with Bill
  Gates"
• _at_fields split
• split _ using space (default) as delimiter
• print "fields0 fields3 fields6\n"
• split2
• Bill love Gates

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join

• The join function allows you to glue strings in a
  list together.
• cat join1
• !/usr/local/bin/perl5 -w
• _at_list qw(Bill Shakespeare dislikes Bill
  Gates)
• line join(" ", _at_list)
• print "line\n"
• join1
• Bill Shakespeare dislikes Bill Gates
• Note that the glue string is not a regular
  expression, just a normal string.