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Introduction to Regular ExpressionsRegular expressions are a powerful way of analysing and retrieving values from text strings. Looking at a full blown regular expression can be intimidating so this tutorial breaks them down into a series of simple examples, easing you into the process. I tend to use the PHP function preg_match when using regular expressions. It takes the form:
Searching for an exact text phraseIf you want to check if an exact text string is within another text string, there are no special regex characters required - you just use the exact text phrase for the regular expression. For example:
Start and end of textIf you're searching for text at the start or end of a text file, use the symbols ^ and $."^The": matches any string that starts with "The"; "of despair$": matches a string that ends in the substring "of despair";
Multiple charactersThe symbols '*', '+', and '?' denote the number of times a character or a sequence of characters may occur. What they mean respectively is: "zero or more", "one or more", and "zero or one". For example:"tu*": matches a string that has the letter t followed by zero or more u's ("t", "tu", "tuuuuu", "tutorial", etc). "tu+": similar but at least one u ("tu", "tuuu", "tut", etc). "tu?": there may or may not be a u. "t?b+$": a possible t followed by one or more u's ending the string.
Or if you want to be more specific on the number of multiple characters, you can specify a range within braces {}.
If you want to quantify a sequence of characters rather than just a single character, put them inside parentheses:
OR operatorThe '|' symbol works as an OR operator: "tips|tutorials": matches a string that has either "tips" or "tutorials" in it."(b|cd)ef": a string that has either "bef" or "cdef". "(a|b)*c": a string that has a sequence of alternating a's and b's ending in a c.
Wild characterA period ('.') is a wild character - it can stand for any single character:"t.*p": matches a string that has a t followed by any number of characters followed by a p ("tip", "tp", "tdfsadfsadsfp", etc). "^.{5}$": a string with exactly 5 characters ("bingo", "blind", "rainy", "asdfe", etc).
Bracket expressionsBracket expressions lets you match a whole range of characters to a single position of a string:"[tu]": matches a string that has either a 't' or a 'u' (that's the same as "t|u"); "[a-d]": a string that has lowercase letters 'a' through 'd' (that's equal to "a|b|c|d" and even "[abcd]"); "^[a-zA-Z]": a string that starts with a letter; "[0-9]%": a string that has a single digit before a percent sign; ",[a-zA-Z0-9]$": a string that ends in a comma followed by an alphanumeric character. Note that inside brackets, all the regex special characters are just ordinary characters - they don't do any of their usual regular expression functions.
Excluding charactersYou can also exclude characters by using a '^' as the first symbol in a bracket expression:"%[^a-zA-Z]%" matches a string with a character that is not a letter between two percent signs). Note - the difference between this application and using ^ at the start of a regular expression which specifies the first character of a string.
Escaping regular expression charactersWhat do you do if you want to check for one of the regular expression special characters "^.[$()|*+?{\" in your text string? You have to escape these characters with a backslash ('\').
Retrieving text using preg_matchIf you want to extract a phrase out of a text string, you use the PHP function preg_match in the following format:
It returns a value of 1 if there is a match to your regular expression, a value of 0 if no match. For example,
Preg_match is really useful for extracting phrases out of a text string. To do this, you specify an array as the third argument (eg - $matchesarray is what I use in the example). You also need to use parenthesizes in your regular expression to specify the sections you want to retrieve. If there's a successful match, $matchesarray is filled with the results of the search. $matchesarray[0] contain the entire text string. $matchesarray[1] contains the text that matched the first captured parenthesized subpattern, and so on. For example, the following regex divides a url into two sections. The first section is "http://" (note the escaping back slash), the second section is whatever comes after:
$matchesarray[0] = "http://www.tipsntutorials.com/" $matchesarray[1] = "http://" $matchesarray[2] = "www.tipsntutorials.com/" Comments
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