Can we use comparison in a regex pattern? I want to check if a recent matched group is greater than a number. This is the pattern: size=(\d+) and I wanna see if (\d+)>200 then return true.
You can't do it with that same syntax, but what you're after is possible with a more complex expression such as:
Greater than 200:
([1-9]\d+|(?!200)[2-9])\d\d
Include 200:
([1-9]\d+|[2-9])\d\d
As you approach other (and larger) numbers, however, it will be unmanagable.
You would do best to match all numbers, then loop through the results to do the > 200 check, and remove ones that don't comply.
Note: The above regular expressions are just to show that matching numbers greater than 200 is possible in pure regex. It is not recommended however, and more complex/variable numbers will be unworkable.
OP mentioned in comments that they would like to match > 30*1024*1024, which is > 31457280
In the name of proving why regex is a pain, I have written the required pattern:
\b((3[1-9][4-9][5-9][7-9][2-9][8-9]\d+)|(3[1-9][4-9][5-9][7-9][3-9]\d{2,})|(3[1-9][4-9][5-9][8-9]\d{3,})|(3[1-9][4-9][6-9]\d{4,})|(3[1-9][5-9]\d{5,})|(3[2-9]\d{6,})|(?!31457280)(4\d{7,}))\b
(not very pretty!)
No, it's not possible, the purpose of the regular expressions is to match expresions not to program with it
I'm new to regex.
I need to validate passwords using php with following password policy using Regex:
Passwords:
Must have minimum 8 characters
Must have 2 numbers
Symbols allowed are : ! # # $ % *
I have tried the following: /^(?=.*\d)(?=.*[A-Za-z])[0-9A-Za-z!##$%]$/
The following matches exactly your requirements: ^(?=.*\d.*\d)[0-9A-Za-z!##$%*]{8,}$
Online demo <<< You don't need the modifiers, they are just there for testing purposes.
Explanation
^ : match begin of string
(?=.*\d.*\d) : positive lookahead, check if there are 2 digits
[0-9A-Za-z!##$%*]{8,} : match digits, letters and !##$%* 8 or more times
$ : match end of string
I would first try and find two numbers, using non-regex (or preg_match_all('[0-9]', ...) >= 2, then validating against:
^[!##$%*a-zA-Z0-9]{8,}$
This should be faster and easier to understand. To do it using only regex sounds you need lookahead which basically scans the expression twice afaik, though I'm not sure of the PHP internals on that one.
Be prepared for a lot of complaints about passwords not being accepted. I personally have a large subset of passwords that wouldn't validate against those restrictions. Also nonsensical passwords like 12345678 would validate, or even 11111111, but not f4#f#faASvCXZr$%%zcorrecthorsebatterystaple.
if(preg_match('/[!##$%*a-zA-Z0-9]{8,}/',$password) && preg_match_all('/[0-9]/',$password) >= 2)
{
// do
}
Full Strong Password Validation With PHP
Min 8 chars long
Min One Digit
Min One Uppercase
Min One Lower Case
Min One Special Chars
/^\S*(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=\S*[\W])[a-zA-Z\d]{8,}\S*$/
Demo here
I'm just getting to know regular expressions, but after doing quite a bit of reading (and learning quite a lot), I still have not been able to figure out a good solution to this problem.
Let me be clear, I understand that this particular problem might be better solved not using regular expressions, but for the sake of brevity let me just say that I need to use regular expressions (trust me, I know there are better ways to solve this).
Here's the problem. I'm given a big file, each line of which is exactly 4 characters long.
This is a regex that defines "valid" lines:
"/^[AB][CD][EF][GH]$/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";
}
My current one works almost perfectly but it misses out any other digits after a character like - or /.
The Original String is something like:
#!012 , #!02/09#!011 #!04-072
My current one works on stuff like:
$viewedResult = preg_replace('/#!([0-9A-Z]{1,4})/', '${1}', $viewedResult);
This would give me something like:
04-072<br />
but I want the "-072" in the Number2 bit like:
04-072
It could also be able to include /072s:
04/072
Any ideas? Remember that there is a #! in front of the number!
You could use something like:
preg_replace('~#!([0-9A-Z][0-9A-Z/-]{0,5})~', '${1}', $viewedResult);
I would split up the regular expression in two, as I think you want to match something like \d{1..4}[-/]?\d{1..4} where the first and second set of digits total 4. It's probably better just to look for (\d+[-/]/d+) and do a check afterwards if the total digits size is correct, e.g. finding all matches of (\d+) in the result, and programmatically check their length.
In general, I would pay close attention to what exactly is allowed input, and what (error) you will get if the input does not match. Splitting things up will make it much easier to show a correct error message, such as "number of digits in mooring spot incorrect" (which is I presume what this is about).
If you want the value of Number2, then use:
Number2\s*=\s*([^;"]*)[;"]
And then use the first capture group.
Stupid question: why not using this regular expression /#!([0-9A-Z/-]{1,4})/ ?
Side note: the {1,4} blocks you will retrieving the value 04-072 (it's 6 chars long).
Hope that helps :)
How can you match the following words by PHP, either by regex/globbing/...?
Examples
INNO, heppeh, isi, pekkep, dadad, mum
My attempt would be to make a regex which has 3 parts:
1st match match [a-zA-Z]*
[a-zA-Z]?
rotation of the 1st match // Problem here!
The part 3 is the problem, since I do not know how to rotate the match.
This suggests me that regex is not the best solution here, since it is too very inefficient for long words.
I think regex are a bad solution. I'd do something with the condition like: ($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~';