CS 497C

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CS 497C Introduction to UNIXLecture 31 -
Filters Using Regular Expressions grep and sed

• Chin-Chih Changchang_at_cs.twsu.edu

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Substitution

• seds strongest feature is substitution, achieved
  with its s (substitute) command.
• It has the following format
• addresss/expression1/string2/flag
• This is how you replace the with a colon
• sed s///g emp.lst head -2
• To check whether substitution is performed, you
  can use the cmp command as follows
• sed s///g emp.lst cmp -l - emp.lst wc -l

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Substitution

• You can perform multiple substitutions with one
  invocation of sed by pressing Enter at the end
  of each instruction, and then close the quote at
  the end
• sed s/ltIgt/ltEMgt/g
• gt s/ltBgt/ltSTRONGgt/g form.html
• You can compress multiple spaces as below
• sed s g emp.lst head -2

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Substitution

• sed /dirctor/s/director/member/ emp.lst
• sed /dirctor/s//member/ emp.lst
• The above command suggests that sed remembers
  the scanned pattern, and stores it in // (2
  frontslashes).
• The // representing an empty (or null) regular
  expression is interpreted to mean that the search
  and substituted patterns are the same. This is
  called the remembered pattern.

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Substitution

• When a pattern in the source string also occurs
  in the replaced string, you can use the special
  character to represent it.
• sed s/director/executive director/ emp.lst
• sed s/director/executive / emp.lst
• These two commands are same. The , known as the
  repeated pattern, expands to the entire source
  string.

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Regular Expressions

• The interval regular expression (IRE) uses the
  escaped pair of curly braces with a single or
  a pair of numbers between them.
• We can use this sequence to display files which
  have write permission set for group
• ls -l grep .\5\w
• The regular expression .\5\w matches five
  characters (.\5\) at the beginning () of the
  line, followed by the pattern (w).

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Regular Expressions

• The \5\ signifies that the previous character
  (.) has to occur five times. The . (dot)
  character is used to match any character.
• The IRE has three forms
• ch\m\ The metacharacter ch can occur m times.
• ch\m,n\ ch can occur between m and n times.
• ch\m,\ ch can occur at least m times.

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Regular Expressions

• We can display the listing for those files that
  have the write bit set either for group or
  others
• ls l grep .\5,8\w
• To locate the people born in 1945 in the sample
  database, use sed as follows
• sed n /.\49\45/p emp.lst
• The tagged regular expression (TRE) uses \( and
  \) to enclose a pattern.

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Regular Expressions

• Suppose you want to replace the words John Wayne
  by Wayne, John. The sed substitution instruction
  will then look like this
• echo John Wayne sed s/\(John\)
  \(Wayne\)/\2, \1/
• Because the TRE remembers a grouped pattern, you
  can look for these repeated words like this
• grep \a-za-za-z\) \1 note

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Regular Expressions

• These are pattern matching options used by grep,
  sed, and perl (Page 441)
• abc match the character string abc.
• zero or more occurrences of previous
  character.
• . match any character except newline.
• . nothing or any number of characters.
• a? match zero or one instance a.
• a match zero or more repetitions of a.

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Regular Expressions

• abcde match any character within the
  brackets.
• a-b match any character within the range a to
  b.
• abcde match any character except those
  within the brackets.
• a-b match any character except those in the
  range a to b.
• match beginning of line, e.g., //.
• lines containing nothing.

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Regular Expressions

• match end of line, e.g., /money./.
• a\2\ match exactly two repetitions of a.
• a\4,\ match four or more repetitions of a.
• a\2, 4\ match between two and four
  repetitions of a.
• \(exp\) expression exp for later referencing
  with \1, \2, etc.
• ab match a or b.