8/11/2019 Regular Expresssion Cheat Sheet: Lecture 3

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Regular expression cheat sheet:

A regular expression tries to match its characters with any sequence of characters somewhere inanother string. There are a lot of special commands used inside of regular expressions, for which this

sheet can be a reference.

Note: When you see Cor A, assume that Cand Acan be replaced with any character. Whenyou see fooor bar, assume that these can be replaced with arbitrary strings of characters.

Matching:Cmatches with the character C

.matches with any other character

^matches the invisible character at the beginning of a string

$matches the invisible character at the end of a string.

Examples:^Phil$matches with Philbut not with PPhil

Philmatches with aaaaaaaPhilaaa

Carriedoes not match with Alex

^CA.GA.$matches with CATGAG

ca.tdoes not match with CAT

Counting:C*matches with any number of occurrences of the character Cin a row.

C+matches with at least one occurrence of the character Cin a row.

C{n,m}matches between n and m occurrences of Cin a row, inclusive.

C{m} matches at most m occurrences of the character Cin a row.

C?mean that Cis optional. t is equivalent to the regular expression C{,!}

Examples:

"*i*c*h*$matches with ###""""""iihhhhhhh

^*e*%*&*e*n+matches with nP

^!'!{(}?$matches with !'!!

^!'!{),}$does not match with !'!!

Grouping:barcreates an equivalence class of characters baand rt will match with any of them.

-foocreates a group of characters. t will only match with the string fooA/Cmatches everything between Aand C.

^barmatches with everything except for the letters baand r.

-foo0barmatches with either fooor bar.

All the signs associated with counting characters can also be placed after equivalence classes and

groups.

Examples:abca/c%t1matches with cat.

8/11/2019 Regular Expresssion Cheat Sheet: Lecture 3

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-Alex0Carrie0Phil0"ichmatches with Phil.

"r{2}i3-ch045?-ch045?matches with "rr"i3ch.

^a/6*will never match with any word made up of only lower case letters.

Escaping:f you want to use a character that is otherwise reserved for some special regular expression command,

you can put a bac!slash -7" in front of it. This is called an escape character. #ince python and theshell both will have confusing behavior when dealing with escape characters, if you want to use themput the character r before the string.

Example:r"7*ichmatches with "*ichbut not with """"""ich.