Lets make a string…
1 | $string = "abcdefghijklmnopqrstuvwxyz0123456789"; |
And perform a really simple regular expression (regex) on it:
1 2 | $string = "abcdefghijklmnopqrstuvwxyz0123456789"; echo preg_match("/abc/", $string); |
This will ouput 1 as it has found it, 0 means it does not exist. Infact if this is all you are doing then you may as well use
1 | strpos() |
and
1 | strstr() |
.
And that’s it!
Before we just matched the string, now we will match the beginning! (using the caret ^).
1 2 3 4 5 6 7 | $string = "abcdefghijklmnopqrstuvwxyz0123456789"; if(preg_match("/^abc/", $string)){ echo 'The string begins with abc'; }else{ echo 'No match found'; } |
It should tell you it starts with ‘abc’ but try matching it to ABC and it wont work, why? Because it’s case-sensitive and we’ll cover that next.
ABC and abc are VERY VERY different in most languages. Infact we will use i as an identifier to decide to use case-insesnitivity…
1 2 3 4 5 6 7 | $string = "abcdefghijklmnopqrstuvwxyz0123456789"; if(preg_match("/^ABC/i", $string)){ echo 'The string begins with abc'; }else{ echo 'No match found'; } |
And BAM! it works!
Remember the ^? well we’re gonna use $ now to match it at the end.
1 2 3 4 5 6 7 | $string = "abcdefghijklmnopqrstuvwxyz0123456789"; if(preg_match("/89$/i", $string)){ echo 'Yes, The string ends with 89'; }else{ echo 'No match found'; } |
And it should state that it ends with 89 and we’ll leave in the i modifier just to be sure.
Remember the ^? Well here’s the list:
If there are any of these in the string then it needs to be escaped with a backslash such as \*
And to escape a backslash we use another, but we also need to escape this so we do it twice: \\\\
Let’s use [ and ] which define character classes!
A Character Class is just a set of characters you wish to match such as:
[abcdefg] or [a-g] both meaning the same thing!
1 2 3 4 | $string = "hut"; // Search for a match echo preg_match("/h[uie]t/", $string, $matches); |
This would return true for “hut”, “hit” and “het” but NOT “hat”!
The symbols ‘*’, ‘+’, and ‘?’ denote the amount of times a string appears.
* = none or more
+ = one or more
? = none or one
1 | "to*" // this matches to "t, toooooo" and toffe" |
In order to find where something doesn’t exist we can use ^ again!
1 2 3 4 5 6 7 8 9 | $string = "abcefghijklmnopqrstuvwxyz0123456789"; // Find where B doesn't exist preg_match("/[^b]/", $string, $matches); // Loop through matches foreach($matches as $key => $value){ echo $key.' -> '.$value; } |
What do we get?
0 -> a
Let’s change something!
1 2 3 4 5 6 7 8 9 | $string = "abcefghijklmnopqrstuvwxyz0123456789"; // Find where g -> o doesn't exist preg_match_all("/[^g-o]/", $string, $matches); // Loop through ALL matches foreach($matches[0] as $key => $value){ echo $value; } |
And bam!
abcefpqrstuvwxyz0123456789!
See now?
This is probably the most used meta-character in regex. This is because it escapes other meta-characters as explained previously.
1 2 3 4 5 6 7 | $string = "This is a [string!]"; preg_match_all("/[\[\]]/", $string, $matches); foreach($matches[0] as $value){ echo $value; } |
And we get [ and ], as really we are just matching [ and ] (“/[[]]/”) but escaped.
Escaping another \ just needs another such as:
\Folder\Files\Documents\etc.txt becomes \\Folder\\Files\\Documents\\etc.txt to do it properly!
(The dot one now (.))!
Let’s match a single character to be missing!
1 2 3 | $string = "The cat sat on the mat"; echo preg_match("/s.t/", $string, $matches); |
You should get 1, as it has made a match! It should also match sit, sot etc…
BUT it does not work with line break characters unless we use the \s flag!
1 2 3 | $string = "The\ncat\nsat\non\nthe\nmat"; echo preg_match_all("/\s/", $string, $matches); |
And guess what, – 4 matches!
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