Regular Expressions

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Regular Expressions

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• A "Regular Expression" (RE):
  • Defines one or more strings of characters

  • Is said to match any string it defines

  • Example:

    /abc/ is an RE that matches "abc"

• A "character": any character except a newline
  • a-z A-Z 0-9 () = ; : ,

  • "special character": a character that represents something other than itself:

    * . [] ? + / \ -

• A "delimiter": a special character marking the start or end of a regular expression; we'll use "/" here.

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Who /uses/ [Rr]eg.lar [Ee]xpre[s*]ions\?

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Regular expressions are used by many Unix utilities:

• sed (Unix stream editor)
• ex (Unix line editor)
• vi (Unix visual editor)
• emacs (general-purpose editor)
• grep (file searching tool)
• awk (pattern scanning and processing language)
• perl (scripting language)

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Using Grep

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• Examples:

          % grep "abc" filename
  

  Prints out all lines in the given file containing "abc" somewhere in them

          % grep -i "abc" filename
  

  Same as above, but ignores case of the desired string

  	% grep -v "abc" filename
  

  Prints out all lines in the given file that do not contain "abc" anywhere in them.

• Quotes are optional around regular expressions that do not contain spaces or shell metacharacters

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Special Characters

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• The period: .
  • Matches any single character

    /a.c/ matches: abc  adc  aec  a=c  a:c

    /x..x/ matches: xaax  xavx  x=kx

• The asterisk: *
  • matches zero or more occurrences of the previous RE
  • This is different than the shell wildcard meaning!

    /ab*c/ matches: ac  abc  abbc  abbbbbbbbbbbbbbbbc

    /a*/ matches: ""  a  aa  aaaaaaaaaa

    /a*b*c*/ matches: ?

    /.*/ matches: ?

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Special Characters (cont.)

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• Square brackets, the Character Class symbol: []
  • Indicates a set of characters, any one of which can match
  • * and $ lose their special meaning
  • ^ at the start means NOT
  • - between characters refers to a range

    /[Mm]ark/ matches: mark  Mark

    /t[aeiou]x/ matches: tax  tex  tix  tox  tux

    /[abc].*/ matches: anthing beginning with a or b or c

    /[a-z][a-z]/ matches: any two-letter lower-case string

    /[a-zA-Z]*/ matches: any word made of letters

    /[^abc].*/ matches: anything starting with something besides a or b or c

    /[a-zA-Z0-9_]*/ matches: ?

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Special Characters (cont.)

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• The caret: ^
  • Outside a character class, means "beginning of line"

    /^T/ matches: all lines starting with T

    /^[0-9]/ matches: ?

• The dollar sign: $
  • Outside of a character class, means "end of line"

    /T$/ matches: all lines ending with T

    /^$/ matches: ?

• The backslash: \
  • Used to quote special characters

    /\./ matches: .

    /a\*b/ matches: a*b

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Regular Expression Rule

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• Longest String Possible:
  • REs will always match the longest string possible starting from the beginning of the line

• Example:

    This (rug) is not what it once was (a long time ago), is it?
  

  /Th.*is/ matches: ?

  /(.*)/ matches: ?

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Full Regular Expressions

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• The plus: +
  • like *, but matches one or more occurrences of the preceding RE

    /ab+c/ matches: abc  abbc  abbbc but not ac

• The question mark: ?
  • matches zero or one occurrences of the previous RE

    /ab?c/ matches: ac  abc

• Logical or: |
  • matches either the RE before or the RE after the vertical bar

    /abc|def/ matches: abc  def

• Parentheses: ( )
  • Can be used to group REs for use with *, ?, +, |, etc.
  • Depending on the program, you may need \( and \) instead

    /ab(c|d)ef/ matches: abcef  abdef