linux crash course appendix a

Regular Expressions• A pattern that matches a set of one or more strings • May be a simple string, or contain wildcard characters or modifiers • Used by programs such as vim, grep, awk,

Appendix A:

Regular Expressions

It’s All Greek to Me

Regular Expressions

• A pattern that matches a set of one or more strings

• May be a simple string, or contain wildcard characters or modifiers

• Used by programs such as vim, grep, awk, and sed

• Not the same as shell expansion

• Characters

– Literals

– Special Characters

• Delimiters

– Mark beginning end of regular expressions – Usually /

– ’ (but not really)

Simple Strings

• Contain no special characters

• Matches only the string

• Ex: /foo/ matches:

– foo

– tomfoolery

– bar.foo.com

Special Characters

• Can match multiple strings

• Represent zero or more characters

• Always match the longest possible string (we’ll see examples in a bit)

• Matches any single character

• Ex: /.ing/

– I was talking

– bling

– he called ingred

• Ex: /spar.ing/

– sparring

– sparking

• Define a character class

• Match any one character in the class

• If a carat (^) is first character in class,

character class matches any character not in class

• Other special characters in class lose

meaning

Brackets con’t

• Ex /[jJ]ustin/ matches justin and Justin

• Ex /[A-Za-z]/ matches any letter

• Ex /[0-9]/ matches any number

• Ex /[^a-z]/ matches anything but

lowercase letters

• Zero or more occurrences of the previous character

• So match any number of characters would

be /.*/

• Ex /t.*ing/

– thing

– this is really annoying

Plus Signs and Question Marks

• Very similar to asterisks, depend on previous

• + matches one or more occurrences (not 0)

• ? Matches zero or one occurrence (no more)

• Ex /2+4?/ matches one or more 2’s

followed by either zero or one 4

– 22224, 2 match

– 4, 244 do not

• Part of the class of extended R.E.

Carets & Dollar Signs

• If a regular expression starts with a ^, the string must be at the beginning of a line

• If a regular expression ends with a $, the string must be at the end of a line

• ^ and $ are referred to as anchors

• Ex /^T.*T$/ matches any line that starts and ends with T

Quoting Special Characters

• If you want to use a special character

literally, put a backslash in front of it

• Ex /and\/or/ matches and/or

• Ex /\\/ matches \

• Ex /\**/ matches any number of asterisks

Longest Match

• Regular expressions match the longest string possible in a line

• Ex I (Justin) like coffee (lots)

• /(.*)/

– Matches (Justin) like coffee (lots)

• /([^)]*)/

– Matches (Justin)

Boolean OR

• You can pattern match for two distinct strings using OR (the pipe)

• Ex /CAT|DOG/

– Matches exactly CAT and exactly DOG

• Simplier expressions can be written just

using a character class

– I.E /a[bc]/ instead of /ab|ac/

• Also part of extended R.E

• You can apply special characters to groups

of characters in parenthesis

• Also called bracketing

• Matches same as unbracketed expression

• But can use modifiers

• Ex /\(duck\)*|\(goose\)/

Using with vim

• Use regular expressions for searching and substituting

• Searching:

– /string or ?string

• Substituting:

– :[g][address]s/string/replace[/g]

– g : global; substitute all lines

– string and replace can be R.E.

– /g : global; replace all occurrences in the line

Using with vim con’t

• [address]

– n : line number

– n[+/-]x : line number plus x lines before or after

– n1,n2 : from line n1 to n2

– : alias for current line

– $ : alias for last line in work buffer

– % : alias for entire work buffer

vim examples

• /^if(

• /end\.$

• :%s/[Jj]ustin/Mr\ Awesome/g

Using with vim con’t

• Ampersand (&)

– Alias for matched string when substituting – Ex: /[A-Z][0-9]/_&_/

• Quoted digit (\n)

– Used with R.E with multiple quoted parts – Can be used to rearrange columns

– Ex: /\([^,]*\), \(.*\)/\2 \1/

Using with grep

• To take advantage of extended regular

expressions, use egrep or grep -E instead

• Use single quote as delimiter

• Ex:

– egrep ’^T.*T$’ myfile

Lists all lines in myfile that begin & end with T