Regular Expressions Theory and Practice

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Regular ExpressionsTheory and Practice

• Jeff Schoolcraft
• MDCFUG 12/13/2005

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Who am I?

• Jeff Schoolcraft
• Senior Architect / Operations Manager at RGII
  Technologies.
• Speaker at Usergroups
• President WinProTeam Vienna Usergroup (.NET)
• TDD Evangelist
• Tool guy

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What can you expect?

• The gist in 60 seconds or less.
• Theory
• Practical Usage
• Best Practices
• Hands On
• A sermon
• Q A

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The Gist

• Regular Expressions (regex) describe patterns in
  strings and are often used for data validation,
  searching and text transformations.

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Theory

• BasicsA regular expression, often called a
  pattern, is an expression that describes a set of
  strings without actually listing its elements.
• Say what?
• The set of strings dog, bog, fog can be
  described by this regular expression (regex)
• bdfog

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Formal Language Theory

• A Regular Language is any language where all
  possible strings of that language can be
  described by a regular expression

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Formal Language Theory (contd)

• Regular expressions consist of constants and
  operators that denote sets of strings and
  operations over these sets, respectively. Given a
  finite alphabet S the following constants are
  defined
• (empty set) Ø denoting the set Ø
• (empty string) e denoting the set e
• (literal character) a in S denoting the set a
• and the following operations
• (concatenation) RS denoting the set aß a in R
  and ß in S . For example "ab", "c""d", "ef"
  "abd", "abef", "cd", "cef".
• (alternation) RS denoting the set union of R and
  S.
• (Kleene star) R denoting the smallest superset
  of R that contains e and is closed under string
  concatenation. This is the set of all strings
  that can be made by concatenating zero or more
  strings in R. For example, "ab", "c" e,
  "ab", "c", "abab", "abc", "cab", "cc", "ababab",
  ... .
• http//en.wikipedia.org/wiki/Regular_expression

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DEMO

• All binary numbers
• All binary numbers that start and end in 1
• All binary number that have 00 any other bits
  followed by 111 and any other bits.

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Practical Usage

• In order of popularity
• Searching
• Much nicer than or
• String Manipulation
• Parsing
• Replacement
• Validation
• Input validation
• Database check constraints

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Best Practices

• The most important thing to remember
• Regular Expressions are greedy
• Make the most explicit match possible
• Just because some implementations allow ? Dont
  fall back on that.

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Hands On
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A Sermon

• Some people develop a religious fascination with
  new tools technologies (design patterns, regex,
  whatever).
• Use the tools that make the most sense for your
  problem/solution.

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Email Validation with REGEX?

• Are you kidding me?
• See email.regex
• Multi-tiered approach, regex to test format, some
  code to test validity of email address.

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Questions?
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Further Resources

• Mastering Regular Expressionshttp//www.oreilly.c
  om/catalog/regex/
• A website, 1000s on google. http//www.regular-e
  xpressions.info/
• Mehttp//thequeue.net/blog/http//regexadvice.co
  m/blogs/jschoolcraft/jeff_at_thequeue.net

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Tools

• The Regulator (http//regex.osherove.com/)
• Expresso (http//www.ultrapico.com/Expresso.htm)