Regex Intro

^[Rr]egular [Ee]xpressions$ Introduction

Vocabulary Regular expression / Regex / Regexp Regex is pronounced Reg (as in register) Ex (as in FedEx) Matching Regex matches a string means it matches in a string

Regular Expressions Composed of two types of characters Metacharacters / Special characters * ? ^ $ . [ ] Literal characters a b c d

Egrep tool Allows you to use Regular Expressions to find words that match Available for Macs, PCs and Linux cat /usr/share/dict/words | egrep ‘…’ See http://regex.info/egrep.html if you don’t have it preinstalled

My first regex cat /usr/share/dict/words | egrep ‘cat’ Matches any words with a ‘c’ followed by an ‘a’ followed by a ‘t’ bobcat cat catwalk scatter Simple regex, only uses Literal chars

Metacharacters: ^ and $ ^ matches the beginning of a line $ matches the end of a line ^cat (start of line followed by ‘c’ then ‘a’ then ‘t’) cat catwalk cat$ (‘c’ followed by ‘a’ then ‘t’ followed by EOL) bobcat cat ^cat$ (start of line followed by ‘c’ then ‘a’ then ‘t’ then EOL) cat

How to read regex Read each character one at a time ^bat Start of line followed by ‘b’ then ‘a’ then ‘t’ rat$ ‘ r’ then ‘a’ then ‘t’ followed by end of line ^dog$ Start of line followed by ‘d’ then ‘o’ then ‘g’ then EOL

More simple regex’s ^ Start of line ^$ Start of line followed by end of line $ End of line ^foot$ Start of line followed by ‘f’ then ‘o’ then ‘o’ then ‘t’ followed by EOL

Character Classes [ ] Matches one of the characters in the [ ] [ae] Matches ‘a’ or ‘e’ [aeiouy] Matches any vowel ^gr[ae]y$ Start of line followed by ‘g’ then ‘r’ then ‘a’ or ‘e’ then ‘y’ followed by end of line grey or gray

Character Classes cont. [Ss] Matches upper or lower case ‘S’ [123456] Matches any of the digits listed [Hh][123456] Matches H1, h2, h3, H4, etc

Special characters in [ ]’s - (dash) references a range [1-6] is the same as [123456] [a-f] is the same as [abcdef] Ranges can be mixed with literals [0-9a-fA-F_!.?] Any digit, upper or lower case ‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’, underscore, exclamation, period or question mark

Negated character class [^ ] ^ inside of [ ] means “not any of these” [^1-6] Any character other than 1, 2, 3, 4, 5, 6 [^a-fA-F] Any character other than A-F (upper or lower) The ^ must be the first character inside [ ] [^c] (Matches anything but ‘c’) [c^] (Matches a ‘c’ or ‘^’)

Translating regex practice List of words that have ‘q’ followed by a character other than ‘u’ q[^u] List of words with ‘f’ followed by an ‘i’ or ‘o’ followed by ‘r’ then ‘e’ f[io]re Line starts with ‘Qu’ or ‘qu’ followed by an ‘e’ followed by any letter between ‘p’ and ‘t’ ^[Qq]ue[p-t]

Metacharacter: . (dot) Matches any character c.t ‘ c’ followed by any character followed by ‘t’ cat cot c8t Period inside of [ ]’s matches a period [a.c] Matches ‘a’, ‘.’ or ‘c’

Periods cont. 03.19.76 Matches ‘03’ followed by a char then ‘19’ then any char then ‘76’ 03-19-76 03/19/76 03.19.76 03 19 76 03 3 19 8 76

Pipes (cont.) Use parenthesis to constrain gre|ay matches ‘gre’ or ‘ay’ gr(e|a)y matches ‘gr’ followed by ‘e’ or ‘a’ then ‘y’

Regex practice Match “First Street” or “1st street” (First|1st) [Ss]treet (Fir|1)st [Ss]treet These are equivalent, which is better? Match “toothbrush” or “hairbrush” (tooth|hair)brush

^ or $ and alternation Be careful when using ^ or $ with alternation ^From|Subject|Date: Start of line followed by From OR Subject OR Date: ^(From|Subject|Date): Start of line followed by ‘From’ or ‘Subject’ or ‘Date’ followed by ‘:’ Safer to use ()’s to group your alternates

Case insensitive match Matches are case sensitive by default [Ff]rom will match From but not FRom Use egrep’s -i option to do a case insensitive match Most languages have a case insensitive match as well

Quantifiers: ? ? metacharacter means optional colou?r matches color or colour ‘ c’ then ‘o’ then ‘l’ then ‘o’ then optionally ‘u’ then ‘r’ Match July or Jul and fourth, 4th and 4 (July|Jul) (fourth|4th|4) July? (fourth|4th|4) July? (fourth|4(th)?)

Quantifiers: + and * + (plus) One or more of the previous item * (star) Zero or more of the previous item b[0-9]*a ba b9999a b999999999999999a

Summary of Quantifiers Minimum Required Maximum to try Meaning ? none 1 zero or one occurrence * none no limit zero or more occurrences + 1 no limit one or more occurrences

Escaping metacharacters Use \ (backslash) to escape metacharacters \. matches ‘.’ . matches any character c.t matches cat c\.t does not match cat $cat$ matches ‘(cat)’ not ‘cat’

More practice Match chat, cat, chart ch?ar?t c[h]?a[r]?t Start of line then M then one or more ‘a’ followed by ‘st’ and zero or more ‘b’ ^M[a]+st[b]* Lines ending with one or more ‘c’ followed by a ‘t’ then zero or one ‘e’ [c]+t[e]*$

More practice ^[Mm][^a-np-z]ney$ Start of line then ‘M’ or ‘m’ then any character not a-n and p-z then ‘ney’ followed by end of line Money, money, m3ney ^be.*(bob|ted)$ Start of line followed by ‘be’ followed by zero or more characters followed by ‘bob’ or ‘ted’ followed by end of line

More practice Match truck, firetruck but not dumptruck ^(fire)?truck$ $0.99, $599.95, $1000.45, $5000 \$[0-9]+(\.[0-9][0-9])?$ 404-555-1212, 404.555.1212, (404) 555-1212 ^[()0-9]+.[0-9]+.[0-9]+$

Regex Intro

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Regex Intro