Regular Expressions

Regular Expressions Regular Expressions are found in Formal Language Theory and can be used to describe a class of languages called regular languages. Regular expression also provide a convenient, compact way of expressing patterns and are particularly useful for describing the token classes (e.g., integer, float, identifier, etc.) to be recognized by the lexical analysis (scanning) phase of compilation.

FAs = Regular Expressions Show concatenation, union and kleene star equivalence using tablet

Regular Expressions in Popular Applications and Languages Unix utility grep – uses regular expressions to perform text searches against files or standard input text. JavaScript, Microsoft ASP and Microsoft .NET - to perform string search and replace operations. Macromedia ColdFusion – input validation using the <CFINPUT> tag (actually processed by client-side JavaScript). MYSQL for performing database searches (e.g., SELECT * FROM table WHERE REGEXP “pattern” ).

Regular Expressions in Popular Applications and Languages Perl – granddaddy of regular expression implementations. Support for regular expressions is a core part of Perl. PHP – supports Perl-compatible regular expression support. Java – Perl-based regular expressions were added in version 1.4 via the java.util.regex.matcher and java.util.regex.pattern classes. Source: Ben Forta. Teach Yourself Regular Expressions in 10 minutes. SAMS, 2004.

egrep regular expressions

egrep regular expressions

Examples To find words of two or more letters that begin and end with y: % egrep ‘^y.*y$’ /usr/dict/words Try: % egrep ‘y.*y’ /usr/dict/words For help with spelling we might type % egrep ‘^rec(ei|ie)ve$’ /usr/dict/words

More examples For help with a crossword puzzle we might type %egrep ‘^s..u.t..e$’ /usr/dict/words To search for all occurrences of the string F(N) in the file ch4.txt, we could type %egrep ‘F\(N\)’ ch4.txt To find two consecutive occurrences of the word the separated by one or more spaces: %egrep ‘(^| )the +the( |$)’ myfile.txt

Practical real-world uses for regular expressions To determine if input is valid for North American Phone Numbers which are made up of a three digit area code (may not start with 0 or 1), followed by a seven-digit number formatted as a three digit prefix followed by a hyphen and a four-digit line number. examples: 248-555-1234 (313) 555-1234 (810)555-1234 Other uses: to check the format of zip codes, SSNs, IP addresses, URLs, email addresses.