How can I get, into an array, all occurrences of this pattern 4321[5-9][7-9]{6} but excluding, for example, the occurrences where there is a digit immediately before the value, or immediately after it?
For instance, 43217999999 should be valid but 143217999999 (note the number 1 at the beginning) should not be valid.
As the first example, 432179999991 shouldn't be valid because of the 1 that it has in the end.
The added difficulty, at least for me, is that I have to parse this in whatever position I can find it inside a string.
The string looks like this, literally:
43217999997 / 543217999999 // 43217999998 _ 43217999999a43216999999-43216999999 arandomword 432159999997
As you would be able to note, it has no standard way of separating the values (I marked in bold the values that would make it invalid, so I shouldn't match those)
My idea right now is something like this:
(\D+|^)(4321[5-9][7-9]{6})(\D+|$)
(\D+|^) meaning that I expect in that position the start of the string or at least one non-digit and (\D+|$) meaning that I expect there the end of the string or at least one non-digit.
That obviously doesn't do what I picture in my head.
I also tried do it in two steps, first:
preg_match_all("/\D+4321[5-9][7-9]{6}\D+|4321[5-9][7-9]{6}\D+|4321[5-9][7-9]{6}$/", $input, $outputArray);
and then:
for($cont = 0; $cont < count($outputArray); $cont++) {
preg_match("/4321[5-9][7-9]{6}/", $outputArray[0][$cont], $outputArray2[]);
}
so I can print
echo "<pre>" . print_r($outputArray2, true) . "</pre>";
but that doesn't let me exclude the ones that have a number before the start of the value (5432157999999 for example), and then, I am not making any progress with my idea.
Thanks in advance for any help.
If you literally want to check if there is no digit before or after the match you can use negative look ahead and look behind.
(?![0-9]) at the end means: "is not followed by 0-9"
(?<![0-9]) at the start means: "is not preceded by 0-9"
See this example https://regex101.com/r/6xbmJk/1
I'm new to writing Regex patterns and I'm struggling to understand why the following line doesn't work.
/^(£)?[0-9]+(?(?=\.[0-9]{2}){0,1}(p)?|$)/
Note: I'm writing this in PHP
I want the code to find £3.10p, but not £3p. Essentially, the letter 'p' can't be allowed unless it is preceded with a decimal point and 2 digits.
EDIT: To clarify, the letter p can be used at the end of the string, however if the string contains a £ and/or a decimal point, the p must be preceded by the point and 2 digits.
More examples of valid inputs:
£3.50
350
£350
234p
Invalid input:
£2p
Could someone please fix this and explain where I've gone wrong here?
Thanks
If 0.50p is allowed, then you can do it like this:
^((£?[0-9]+)(?!p)|([0-9]+p?))?(?<!p)(\.[0-9]{2})?p?$
Regex saved with all your examples here: https://regex101.com/r/rE1bT9/3
Try this:
/^(?(?=£)(£\d+\.\d{2}p?|£\d+)|\d+p?)$/
You can test it here:
https://regex101.com/r/mG8kR0/1
It is unclear how your valid sample "234p" matches your rule "p is allowed if there are at least two digits and a point". However, in your question you are using positive lookahead, this seems an overhead here.
Your rule for p may be written as: (\.[0-9]{2}p?)
So over all, you just need: /^(£)?[0-9]+(\.[0-9]{2}p?)$/
And if you allow "234p" also, just make the period optional: /^(£)?[0-9]+(\.?[0-9]{2}p?)$/
Try it out here: http://www.regexr.com/
The latter regex gives positive feedback to all your valid samples, and it denies the invalid input. It is unclear what should happen if there are only two digits, and if it is important to catch some pieces, there should be more brackets.
How about:
/^(?:£?[0-9]+(?:\.[0-9]{2})?|[0-9]+p?)$)/
I'm just getting to know regular expressions, but after doing quite a bit of reading (and learning quite a lot), I still have not been able to figure out a good solution to this problem.
Let me be clear, I understand that this particular problem might be better solved not using regular expressions, but for the sake of brevity let me just say that I need to use regular expressions (trust me, I know there are better ways to solve this).
Here's the problem. I'm given a big file, each line of which is exactly 4 characters long.
This is a regex that defines "valid" lines:
"/^[AB][CD][EF][GH]$/m"
In english, each line has either A or B at position 0, either C or D at position 1, either E or F at position 2, and either G or H at position 3. I can assume that each line will be exactly 4 characters long.
What I'm trying to do is given one of those lines, match all other lines that contain 2 or more common characters.
The below example assumes the following:
$line is always a valid format
BigFileOfLines.txt contains only valid lines
Example:
// Matches all other lines in string that share 2 or more characters in common
// with "$line"
function findMatchingLines($line, $subject) {
$regex = "magic regex I'm looking for here";
$matchingLines = array();
preg_match_all($regex, $subject, $matchingLines);
return $matchingLines;
}
// Example Usage
$fileContents = file_get_contents("BigFileOfLines.txt");
$matchingLines = findMatchingLines("ACFG", $fileContents);
/*
* Desired return value (Note: this is an example set, there
* could be more or less than this)
*
* BCEG
* ADFG
* BCFG
* BDFG
*/
One way I know that will work is to have a regex like the following (the following regex would only work for "ACFG":
"/^(?:AC.{2}|.CF.|.{2}FG|A.F.|A.{2}G|.C.G)$/m"
This works alright, performance is acceptable. What bothers me about it though is that I have to generate this based off of $line, where I'd rather have it be ignorant of what the specific parameter is. Also, this solution doesn't scale terrible well if later the code is modified to match say, 3 or more characters, or if the size of each line grows from 4 to 16.
It just feels like there's something remarkably simple that I'm overlooking. Also seems like this could be a duplicate question, but none of the other questions I've looked at really seem to address this particular problem.
Thanks in advance!
Update:
It seems that the norm with Regex answers is for SO users to simply post a regular expression and say "This should work for you."
I think that's kind of a halfway answer. I really want to understand the regular expression, so if you can include in your answer a thorough (within reason) explanation of why that regular expression:
A. Works
B. Is the most efficient (I feel there are a sufficient number of assumptions that can be made about the subject string that a fair amount of optimization can be done).
Of course, if you give an answer that works, and nobody else posts the answer *with* a solution, I'll mark it as the answer :)
Update 2:
Thank you all for the great responses, a lot of helpful information, and a lot of you had valid solutions. I chose the answer I did because after running performance tests, it was the best solution, averaging equal runtimes with the other solutions.
The reasons I favor this answer:
The regular expression given provides excellent scalability for longer lines
The regular expression looks a lot cleaner, and is easier for mere mortals such as myself to interpret.
However, a lot of credit goes to the below answers as well for being very thorough in explaining why their solution is the best. If you've come across this question because it's something you're trying to figure out, please give them all a read, helped me tremendously.
Why don't you just use this regex $regex = "/.*[$line].*[$line].*/m";?
For your example, that translates to $regex = "/.*[ACFG].*[ACFG].*/m";
This is a regex that defines "valid" lines:
/^[A|B]{1}|[C|D]{1}|[E|F]{1}|[G|H]{1}$/m
In english, each line has either A or B at position 0, either C or D
at position 1, either E or F at position 2, and either G or H at
position 3. I can assume that each line will be exactly 4 characters
long.
That's not what that regex means. That regex means that each line has either A or B or a pipe at position 0, C or D or a pipe at position 1, etc; [A|B] means "either 'A' or '|' or 'B'". The '|' only means 'or' outside of character classes.
Also, {1} is a no-op; lacking any quantifier, everything has to appear exactly once. So a correct regex for the above English is this:
/^[AB][CD][EF][GH]$/
or, alternatively:
/^(A|B)(C|D)(E|F)(G|H)$/
That second one has the side effect of capturing the letter in each position, so that the first captured group will tell you whether the first character was A or B, and so on. If you don't want the capturing, you can use non-capture grouping:
/^(?:A|B)(?:C|D)(?:E|F)(?:G|H)$/
But the character-class version is by far the usual way of writing this.
As to your problem, it is ill-suited to regular expressions; by the time you deconstruct the string, stick it back together in the appropriate regex syntax, compile the regex, and do the test, you would probably have been much better off just doing a character-by-character comparison.
I would rewrite your "ACFG" regex thus: /^(?:AC|A.F|A..G|.CF|.C.G|..FG)$/, but that's just appearance; I can't think of a better solution using regex. (Although as Mike Ryan indicated, it would be better still as /^(?:A(?:C|.E|..G))|(?:.C(?:E|.G))|(?:..EG)$/ - but that's still the same solution, just in a more efficiently-processed form.)
You've already answered how to do it with a regex, and noted its shortcomings and inability to scale, so I don't think there's any need to flog the dead horse. Instead, here's a way that'll work without the need for a regex:
function findMatchingLines($line) {
static $file = null;
if( !$file) $file = file("BigFileOfLines.txt");
$search = str_split($line);
foreach($file as $l) {
$test = str_split($l);
$matches = count(array_intersect($search,$test));
if( $matches > 2) // define number of matches required here - optionally make it an argument
return true;
}
// no matches
return false;
}
There are 6 possibilities that at least two characters match out of 4: MM.., M.M., M..M, .MM., .M.M, and ..MM ("M" meaning a match and "." meaning a non-match).
So, you need only to convert your input into a regex that matches any of those possibilities. For an input of ACFG, you would use this:
"/^(AC..|A.F.|A..G|.CF.|.C.G|..FG)$/m"
This, of course, is the conclusion you're already at--so good so far.
The key issue is that Regex isn't a language for comparing two strings, it's a language for comparing a string to a pattern. Thus, either your comparison string must be part of the pattern (which you've already found), or it must be part of the input. The latter method would allow you to use a general-purpose match, but does require you to mangle your input.
function findMatchingLines($line, $subject) {
$regex = "/(?<=^([AB])([CD])([EF])([GH])[.\n]+)"
+ "(\1\2..|\1.\3.|\1..\4|.\2\3.|.\2.\4|..\3\4)/m";
$matchingLines = array();
preg_match_all($regex, $line + "\n" + $subject, $matchingLines);
return $matchingLines;
}
What this function does is pre-pend your input string with the line you want to match against, then uses a pattern that compares each line after the first line (that's the + after [.\n] working) back to the first line's 4 characters.
If you also want to validate those matching lines against the "rules", just replace the . in each pattern to the appropriate character class (\1\2[EF][GH], etc.).
People may be confused by your first regex. You give:
"/^[A|B]{1}|[C|D]{1}|[E|F]{1}|[G|H]{1}$/m"
And then say:
In english, each line has either A or B at position 0, either C or D at position 1, either E or F at position 2, and either G or H at position 3. I can assume that each line will be exactly 4 characters long.
But that's not what that regex means at all.
This is because the | operator has the highest precedence here. So, what that regex really says, in English, is: Either A or | or B in the first position, OR C or | or D in the first position, OR E or | or F in the first position, OR G or '|orH` in the first position.
This is because [A|B] means a character class with one of the three given characters (including the |. And because {1} means one character (it is also completely superfluous and could be dropped), and because the outer | alternate between everything around it. In my English expression above each capitalized OR stands for one of your alternating |'s. (And I started counting positions at 1, not 0 -- I didn't feel like typing the 0th position.)
To get your English description as a regex, you would want:
/^[AB][CD][EF][GH]$/
The regex will go through and check the first position for A or B (in the character class), then check C or D in the next position, etc.
--
EDIT:
You want to test for only two of these four characters matching.
Very Strictly speaking, and picking up from #Mark Reed's answer, the fastest regex (after it's been parsed) is likely to be:
/^(A(C|.E|..G))|(.C(E)|(.G))|(..EG)$/
as compared to:
/^(AC|A.E|A..G|.CE|.C.G|..EG)$/
This is because of how the regex implementation steps through text. You first test if A is in the first position. If that succeeds, then you test the sub-cases. If that fails, then you're done with all those possible cases (or which there are 3). If you don't yet have a match, you then test if C is in the 2nd position. If that succeeds, then you test for the two subcases. And if none of those succeed, you test, `EG in the 3rd and 4th positions.
This regex is specifically created to fail as fast as possible. Listing each case out separately, means to fail, you would have test 6 different cases (each of the six alternatives), instead of 3 cases (at a minimum). And in cases of A not being the first position, you would immediately go to test the 2nd position, without hitting it two more times. Etc.
(Note that I don't know exactly how PHP compiles regex's -- it's possible that they compile to the same internal representation, though I suspect not.)
--
EDIT: On additional point. Fastest regex is a somewhat ambiguous term. Fastest to fail? Fastest to succeed? And given what possible range of sample data of succeeding and failing rows? All of these would have to be clarified to really determine what criteria you mean by fastest.
Here's something that uses Levenshtein distance instead of regex and should be extensible enough for your requirements:
$lines = array_map('rtrim', file('file.txt')); // load file into array removing \n
$common = 2; // number of common characters required
$match = 'ACFG'; // string to match
$matchingLines = array_filter($lines, function ($line) use ($common, $match) {
// error checking here if necessary - $line and $match must be same length
return (levenshtein($line, $match) <= (strlen($line) - $common));
});
var_dump($matchingLines);
I bookmarked the question yesterday in the evening to post an answer today, but seems that I'm a little late ^^ Here is my solution anyways:
/^[^ACFG]*+(?:[ACFG][^ACFG]*+){2}$/m
It looks for two occurrences of one of the ACFG characters surrounded by any other characters. The loop is unrolled and uses possessive quantifiers, to improve performance a bit.
Can be generated using:
function getRegexMatchingNCharactersOfLine($line, $num) {
return "/^[^$line]*+(?:[$line][^$line]*+){$num}$/m";
}
Alright, so I want the user to be able to enter every character from A-Z and every number from 0-9, but I don't want them entering "special characters".
Code:
if (preg_match("/^[a-zA-Z0-9]$/", $user_name)) {
#Stuff
}
How is it possible for it to check all of the characters given, and then check if those were matched? I've tried preg_match_all(), but I didn't honestly understand much of it.
Like if a user entered "FaiL65Mal", I want it to allow it and move on. But if they enter "Fail{]^7(,", I want it to appear with an error.
You just need a quantifier in your regex:
Zero or more characters *:
/^[a-zA-Z0-9]*$/
One or more characters +:
/^[a-zA-Z0-9]+$/
Your regex as is will only match a string with exactly one character that is either a letter or number. You want one of the above options for zero or more or one or more, depending on if you want to allow or reject the empty string.
Your regular expression needs to be changed to
/^[a-zA-Z0-9]{1,8}$/
For usernames between 1 and 8 characters. Just adjust the 8 to the appropriate number and perhaps the 1.
Currently your expression matches one character
Please keep in mid that preg_match() and other preg_*() functions aren't reliable because they return either 0 or false on fail, so a simple if won't throw on error.
Consider using T-Regx:
if (pattern(('^[a-zA-Z0-9]{1,8}$')->matches($input))
{
// Matches! :)
}
How can you match the following words by PHP, either by regex/globbing/...?
Examples
INNO, heppeh, isi, pekkep, dadad, mum
My attempt would be to make a regex which has 3 parts:
1st match match [a-zA-Z]*
[a-zA-Z]?
rotation of the 1st match // Problem here!
The part 3 is the problem, since I do not know how to rotate the match.
This suggests me that regex is not the best solution here, since it is too very inefficient for long words.
I think regex are a bad solution. I'd do something with the condition like: ($word == strrev($word)).
Regexs are not suitable for finding palindromes of an arbitrary length.
However, if you are trying to find all of the palindromes in a large set of text, you could use regex to find a list of things that might be palindromes, and then filter that list to find the words that actually are palindromes.
For example, you can use a regex to find all words such that the first X characters are the reverse of the last X characters (from some small fixed value of X, like 2 or 3), and then run a secondary filter against all the matches to see if the whole word is in fact a palindrome.
In PHP once you get the string you want to check (by regex or split or whatever) you can just:
if ($string == strrev($string)) // it's a palindrome!
i think this regexp can work
$re = '~([a-z])(.?|(?R))\1~';