ITEC 325 2011fall ibarland

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lect04b-regexps
input validation
functions; miscellania

PHP regular expressions

      echo preg_match( '/row/', 'How now, brown cow?' );
      echo preg_match( '/.ow/', 'How now, brown cow?' );
      echo preg_match( '/n[oe]w/', 'How now, brown cow?' );
      echo preg_match( '/rows?/', 'How now, brown cow, eh?' );
      echo preg_match( '/rows?/', 'How now, brown cows, eh?' );
      echo preg_match( '/rows?/', 'How now, brown cowss, eh?' );

      echo preg_match_all( '/ow/', 'How now, brown cow?' );
      echo preg_match_all( '/rows?/', '\"How now, brown cow?\", he crows.' );

      echo preg_match( '/[0-9]*/', '00047' );
      echo preg_match( '/\\d*/',   '00047' );
      echo preg_match( '/[0-9]+/', '00047' );
    

Note: the name “preg” comes from "Perl compatible"; earlier PHP used the POSIX regexp's but PHP decided to deprecate that.

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 2011.Sep.23 (Fri)

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