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

Components 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)

Periods Matches any single character Ex: /.ing/ –I was talking –bling –he called ingred Ex: /spar.ing/ –sparring –sparking

Brackets 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

Asterisks 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.

Grouping 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