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lect05b-regexps
regular expressions

PHP regular expressions

PHP regular-expression matching: In php, regular expressions are strings delimited by a special character (usually /). Some very quick examples: lect05b-regexp-examples.php

      echo preg_match( '/abcd/', 'abcd' );
      echo preg_match( '/abcd/', 'azcd' );      // false

      echo preg_match( '/a..d/', 'azcd' );      // . matches any single character (besides newline, null)

      echo preg_match( '/ab*cd/', 'abbbbcd' );  // b*  matches 0-or-more-b's.
      echo preg_match( '/ab*cd/', 'abcd' );  
      echo preg_match( '/ab*cd/', 'acd'  ); 

      echo preg_match( '/ab+cd/', 'abbbbcd' );  // b+  matches 1-or-more-b's.
      echo preg_match( '/ab+cd/', 'abcd' );  
      echo preg_match( '/ab+cd/', 'acd'  );     // false

      echo preg_match( '/ab.*cd/', 'abcd' );
      echo preg_match( '/ab.*cd/', 'abXd' );               // .* and .+ are a common patterns.
      echo preg_match( '/ab.*cd/', 'abBlahBlahBlahcd' );
      echo preg_match( '/ab.+cd/', 'abcd' );               // false
      echo preg_match( '/ab.+cd/', 'abXcd' );
      echo preg_match( '/ab.+cd/', 'abBlahBlahBlahcd' );   // false

      
      echo preg_match( '/ab?cd/', 'acd' );  // b?  matches 0-or-1 b
      echo preg_match( '/ab?cd/', 'abcd' );  
      echo preg_match( '/ab?cd/', 'abbbbcd'  );     // false

      echo preg_match( '/(ab)*cd/', 'ababababcd' );  // parens do grouping
      echo preg_match( '/(ab)*cd/', 'abbbbcd' );  // false


      // WARNING: preg_match looks to see if the string *contains* a match!
      echo preg_match( '/row/', 'How now, brown cow?' );  // true
      echo preg_match( '/.ow/', 'How now, brown cow?' );  // true

      // Use "^" to specify the start-of-string, and "$" to specify end-of-string.
      echo preg_match( '/^.ow$/', 'How now, brown cow?' );  // false
      echo preg_match( '/^.ow$/', 'Zow' );
      echo preg_match( '/^.ow/', 'Zowee' );
      echo preg_match( '/w.e$/', 'Wowee Zowee' );

      
      echo preg_match( '/[WZ]ow/', 'Wowee Zowee' );  // square-brackets match any one character from the set
      echo preg_match( '/[WZ]ow/', 'Yow' );          // false
      echo preg_match( '/[W-Z]ow/', 'Yowee' );       // square-brackets can contain a *range*
      echo preg_match( '/ab[0-9]+de/', 'ab789de' );


      echo preg_match( '/[0-9]*/', '00047' );        // Beware: matching just a * expression (w/o ^,$)!
    

Your task: What is a regular expression to match...

• The name "Jon"?
• The name "Jon" or "Jonathon"?
• Any name starting "Jon"?
• Any name containing "Jon"?
• A hyphenated name.
• A doubly-hyphenated name.
• A name starting with a lower-case letter.
• A name ending with a digit.
• A name ending with a roman numeral.
• A name not starting with a capital letter.

Atomic regexps

• ^ : matches start-of-string
• $ : matches end-of-string
• . : matches any one character
• Most characters match themselves (e.g. 'b'); you can escape special characters with backslash.
• Character classes:
  • [[:space:]]
  • [[:alpha:]]
  • [[:upper:]]
  • [[:lower:]]
  • [[:digit:]] matches '0'..'9'
  • [[:punct:]] One of: “.”   “!”   “?”   “:”   “;”   “?” “"”   “'”
  • […] — build your own character set, e.g. [aeiou] matches any one lower-case vowel.
    Use ^ just inside the [ to negate the set. (Note that this overrides the beginning-of-string meaning of ^; it's clear from context).
  Each of these match one character; they are often used in conjunction with * and + below.

Compound regexps

• (α) — same as α; used to dis-ambiguate precedence.
• α* — 0 or more occurrences of α
• α+ — 1 or more occurrences of α
• α? — 0 or 1 occurrence of α
• α{…} — you specify a range of #repetitions of α (not very common)
• α|β — α OR β. This is the one low-priority regexp operator: ab|cd is interpreted as (ab)|(cd), not a(b|c)d. Compare that with any of the others: ab*c is interpreted as a(b*)c, not (ab)*c.

NOTE: No trailing “g” allowed, as in javascript!
(There are other trailing modifiers however — e.g. i for case-insensitive, and more.)

• preg_match($pat,$target) (also, note: preg_match_all — not as common, but it motivates why preg_match's output is 0,1 and not a raw boolean).

  Warning: preg_match_all("/x/","abc",$results) will always have $results be non-empty — it will be an array containing an empty array. (In particular, the condition of if ($results) … will always be true.) You can instead use preg_match_all("/x/","abc",$results,PREG_SET_ORDER) which will have zero elements for zero matches (and if ($results) … will behave as you want). Each element of $results will still be an another entire array though -- the full match plus any sub-patterns.

• preg_replace($pat,$target,$new)
• split($pat,$target) Returns an array (indexed numerically from 0)
• trim — nothing to do with regular expressions, but a function you probably want to call on most user-entered text.

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©2011, Ian Barland, Radford University Last modified 2012.Feb.17 (Fri)

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