Some time in the near future I will need to implement a cross-language word count, or if that is not possible, a cross-language character count.
By word count I mean an accurate count of the words contained within the given text, taking the language of the text. The language of the text is set by a user, and will be assumed to be correct.
By character count I mean a count of the "possibly in a word" characters contained within the given text, with the same language information described above.
I would much prefer the former count, but I am aware of the difficulties involved. I am also aware that the latter count is much easier, but very much prefer the former, if at all possible.
I'd love it if I just had to look at English, but I need to consider every language here, Chinese, Korean, English, Arabic, Hindi, and so on.
I would like to know if Stack Overflow has any leads on where to start looking for an existing product / method to do this in PHP, as I am a good lazy programmer*
A simple test showing how str_word_count with set_locale doesn't work, and a function from php.net's str_word_count page.
*http://blogoscoped.com/archive/2005-08-24-n14.html
Counting chars is easy:
echo strlen('一个有十的字符的句子'); // 30 (WRONG!)
echo strlen(utf8_decode('一个有十的字符的句子')); // 10
Counting words is where things start to get tricky, specially for Chinese, Japanese and other languages that don't use spaces (or other common "word boundary" characters) as word separators. I don't speak Chinese and I don't understand how word counting works in Chinese, so you'll have to educate me a bit - what makes a word in these languages? Is it any specific char or set of chars? I remember reading something related to how hard it was to identify Japanese words in T9 writing but can't find it anymore.
The following should correctly return the number of words in languages that use spaces or punctuation chars as words separators:
count(preg_split('~[\p{Z}\p{P}]+~u', $string, null, PREG_SPLIT_NO_EMPTY));
A quick trick if you only want approximate and not exact words is
<?php echo count(explode(' ',$string)); ?>
It works by counting spaces in just any language. I have used this for a translator script. Again it will not count exact words but give approximate words in a para.
Well, try:
<?
function count_words($str){
$words = 0;
$str = eregi_replace(" +", " ", $str);
$array = explode(" ", $str);
for($i=0;$i < count($array);$i++)
{
if (eregi("[0-9A-Za-zÀ-ÖØ-öø-ÿ]", $array[$i]))
$words++;
}
return $words;
}
echo count_words('This is the second one , it will count wrong as well" , it will count 12 instead of 11 because the comma is counted too.');
?>
Related
I need to change the decimal separator in a given string that has numbers in it.
What RegEx code can ONLY select the thousands separator character in the string?
It need to only select, when there is number around it. For example only when 123,456 I need to select and replace ,
I'm converting English numbers into Persian (e.g: Hello 123 becomes Hello ۱۲۳). Now I need to replace the decimal separator with Persian version too. But I don't know how I can select it with regex. e.g. Hello 121,534 most become Hello ۱۲۱/۵۳۴
The character that needs to be replaced is , with /
Use a regular expression with lookarounds.
$new_string = preg_replace('/(?<=\d),(?=\d)/', '/', $string);
DEMO
(?<=\d) means there has to be a digit before the comma, (?=\d) means there has to be a digit after it. But since these are lookarounds, they're not included in the match, so they don't get replaced.
According to your question, the main problem you face is to convert the English number into the Persian.
In PHP there is a library available that can format and parse numbers according to the locale, you can find it in the class NumberFormatter which makes use of the Unicode Common Locale Data Repository (CLDR) to handle - in the end - all languages known to the world.
So converting a number 123,456 from en_UK (or en_US) to fa_IR is shown in this little example:
$string = '123,456';
$float = (new NumberFormatter('en_UK', NumberFormatter::DECIMAL))->parse($string);
var_dump(
(new NumberFormatter('fa_IR', NumberFormatter::DECIMAL))->format($float)
);
Output:
string(14) "۱۲۳٬۴۵۶"
(play with it on 3v4l.org)
Now this shows (somehow) how to convert the number. I'm not so firm with Persian, so please excuse if I used the wrong locale here. There might be options as well to tell which character to use for grouping, but for the moment for the example, it's just to show that conversion of the numbers is taken care of by existing libraries. You don't need to re-invent this, which is even a sort of miss-wording, this isn't anything a single person could do, or at least it would be sort of insane to do this alone.
So after clarifying on how to convert these numbers, question remains on how to do that on the whole text. Well, why not locate all the potential places looking for and then try to parse the match and if successful (and only if successful) convert it to the different locale.
Luckily the NumberFormatter::parse() method returns false if parsing did fail (there is even more error reporting in case you're interested in more details) so this is workable.
For regular expression matching it only needs a pattern which matches a number (largest match wins) and the replacement can be done by callback. In the following example the translation is done verbose so the actual parsing and formatting is more visible:
# some text
$buffer = <<<TEXT
it need to only select , when there is number around it. for example only
when 123,456 i need to select and replace "," I'm converting English
numbers into Persian (e.g: "Hello 123" becomes "Hello ۱۲۳"). now I need to
replace the Decimal separator with Persian version too. but I don't know how
I can select it with regex. e.g: "Hello 121,534" most become
"Hello ۱۲۱/۵۳۴" The character that needs to be replaced is , with /
TEXT;
# prepare formatters
$inFormat = new NumberFormatter('en_UK', NumberFormatter::DECIMAL);
$outFormat = new NumberFormatter('fa_IR', NumberFormatter::DECIMAL);
$bufferWithFarsiNumbers = preg_replace_callback(
'(\b[1-9]\d{0,2}(?:[ ,.]\d{3})*\b)u',
function (array $matches) use ($inFormat, $outFormat) {
[$number] = $matches;
$result = $inFormat->parse($number);
if (false === $result) {
return $number;
}
return sprintf("< %s (%.4f) = %s >", $number, $result, $outFormat->format($result));
},
$buffer
);
echo $bufferWithFarsiNumbers;
Output:
it need to only select , when there is number around it. for example only
when < 123,456 (123456.0000) = ۱۲۳٬۴۵۶ > i need to select and replace "," I'm converting English
numbers into Persian (e.g: "Hello < 123 (123.0000) = ۱۲۳ >" becomes "Hello ۱۲۳"). now I need to
replace the Decimal separator with Persian version too. but I don't know how
I can select it with regex. e.g: "Hello < 121,534 (121534.0000) = ۱۲۱٬۵۳۴ >" most become
"Hello ۱۲۱/۵۳۴" The character that needs to be replaced is , with /
Here the magic is just two bring the string parts into action with the number conversion by making use of preg_replace_callback with a regular expression pattern which should match the needs in your question but is relatively easy to refine as you define the whole number part and false positives are filtered thanks to the NumberFormatter class:
pattern for Unicode UTF-8 strings
|
(\b[1-9]\d{0,2}(?:[ ,.]\d{3})*\b)u
| | |
| grouping character |
| |
word boundary -----------------+
(play with it on regex101.com)
Edit:
To only match the same grouping character over multiple thousand blocks, a named reference can be created and referenced back to it for the repetition:
(\b[1-9]\d{0,2}(?:(?<grouping_char>[ ,.])\d{3}(?:(?&grouping_char)\d{3})*)?\b)u
(now this get's less easy to read, get it deciphered and play with it on regex101.com)
To finalize the answer, only the return clause needs to be condensed to return $outFormat->format($result); and the $outFormat NumberFormatter might need some more configuration but as it is available in the closure, this can be done when it is created.
(play with it on 3v4l.org)
I hope this is helpful and opens up a broader picture to not look for solutions only because hitting a wall (and only there). Regex alone most often is not the answer. I'm pretty sure there are regex-freaks which can give you a one-liner which is pretty stable, but the context of using it will not be very stable. However not saying there is only one answer. Instead bringing together different levels of doings (divide and conquer) allows to rely on a stable number conversion even if yet still unsure on how to regex-pattern an English number.
You can write a regex to capture numbers with thousand separator, and then aggregate the two numeric parts with the separator you want :
$text = "Hello, world, 121,534" ;
$pattern = "/([0-9]{1,3}),([0-9]{3})/" ;
$new_text = preg_replace($pattern, "$1X$2", $text); // replace comma per 'X', keep other groups intact.
echo $new_text ; // Hello, world, 121X534
In PHP you can do that using str_replace
$a="Hello 123,456";
echo str_replace(",", "X", $a);
This will return: Hello 123X456
I am trying to detect with regex, strings that have a pattern of {any_number}{x-}{large|medium|small} for a site with clothing I am building in PHP.
I have managed to match the sizes against a preconfigured set of strings by using:
$searchFor = '7x-large';
$regex = '/\b'.$searchFor.'\b/';
//Basically, it's finding the letters
//surrounded by a word-boundary (the \b bits).
//So, to find the position:
preg_match($regex, $opt_name, $match, PREG_OFFSET_CAPTURE);
I even managed to detect weird sizes like 41 1/2 with regex, but I am not an expert and I am having a hard time on this.
I have come up with
preg_match("/^(?<![\/\d])([xX\-])(large|medium|small)$/", '7x-large', $match);
but it won't work.
Could you pinpoint what I am doing wrong?
It sounds like you also want to match half sizes. You can use something like this:
$theregex = '~(?i)^\d+(?:\.5)?x-(?:large|medium|small)$~';
if (preg_match($theregex, $yourstring,$m)) {
// Yes! It matches!
// the match is $m[0]
}
else { // nah, no luck...
}
Note that the (?i) makes it case-insensitive.
This also assumes you are validating that an entire string conforms to the pattern. If you want to find the pattern as a substring of a larger string, remove the ^ and $ anchors:
$theregex = '~(?i)\d+(?:\.5)?x-(?:large|medium|small)~';
Look at the specification you have and build it up piece by piece. You want "{any_number}{x-}{large|medium|small}".
"{any_number}" would be \d+. This does not allow fractional numbers such as 12.34, but the question does not specify whether they are required.
"{x-}" is a simple string x-
"{large|medium|small}" is a choice between three alternatives large|medium|small.
Joining the pieces together gives \d+x-(large|medium|small). Note the brackets around the alternation, without then the expression would be interpreted as (\d+x-large)|medium|small.
You mention "weird sizes like 41 1/2" but without specifying how "weird" the number to be matched are. You need a precise specification of what you include in "weird" before you can extend the regular expression.
As this question, I can split strings that includes upper cases like this:
function splitAtUpperCase($string){
return preg_replace('/([a-z0-9])?([A-Z])/','$1 $2',$string);
}
$string = 'setIfUnmodifiedSince';
echo splitAtUpperCase($string);
Output is "set If Unmodified Since"
But I need some modification:
That code snippet doesn't handle the cases, when these characters exist in string: ÇÖĞŞÜİ. I don't want to transliterate the characters. Then I lose meaning of word. I need to use some UTF characters. That code makes "HereÇonThen" to "HereÇon Then"
I also don't want to split uppercase abbreviations. If word is "IKnowYouWillComeASAPHere" I need it to be converted to "I Know You Will Come ASAP Here"
Don't explode if all letters are uppercase. Like "DONTCOMEHERE"
Explode also numeric values. "Before2013ends" to "Before 2013 ends"
Explode if first character is hash key (#).
cases and expected results
"comeHEREtomorrow" => "come HERE tomorrow"
"KissYouTODAY" => "kiss you TODAY"
"comeÜndeHere" => "come Ünde Here"
"NEVERSAYIT" => "NEVERSAYIT"
"2013willCome" => "2013 will Come"
"Before2013ends" => "Before 2013 ends"
"IKnowThat" => "I Know That"
"#whatiknow" => "# whatiknow"
For these cases I use subsequent str_replace operations. I look for a short solution that doesn't make too much for loops to check the words. It would be better to have it as preg_replace or etc. if possible.
Edit: Anyone can try his solution by changing convert function inside this PHP fiddle: http://ideone.com/9gajZ8
/([[:lower:][:digit:]])?([[:upper:]]+)/u should do it.
Here /u is used for Unicode characters. and ([[:upper:]]+) is used for Sequence of upper cased letters.
Note. Case of a letter depends on the character set you are using.
Some notes:
Use Unicode properties to search for upper-case & lower-case letters (and even title-case ones, f.ex. Dž Lj Nj Dz)
comeHEREtomorrow & IKnowThat won't work with one method, until you use some dictionaries to find exact words.
Because if you want to translate comeHEREtomorrow as come HERE tomorrow, IKnowThat will be IK now That (or even IK now T hat);
And if you want to translate IKnowThat as I Know That, comeHEREtomorrow will be come H E R E tomorrow
My solution: http://ideone.com/oALyTo (excludes non-letter & non-number charaters)
Well, I matched all of your test cases, but I still don't think it's a good solution. (One of the few flaws in test driven design).
I took a slightly different approach. Instead of trying to write a regular expression for what the place between a word should look like, I wrote a regular expression that looks for everything that apparently is a word, and then imploded.
function convert($keyword) {
$wResult = preg_match_all('/(^I|[[:upper:]]{2,}|[[:upper:]][[:lower:]]*|[[:lower:]]+|\d+|#)/u', $keyword, $matches);
return implode(' ',$matches[0]);
}
As you can see, this is what I decided qualified as a word:
^I A capital I at the beginning of the string. Break point: Icons.
[[:upper:]]{2,} Consecutive capitals. Break Point: WellIKnowThat
[[:upper:]][[:lower:]]* A single Capital followed by some lower case letters
[[:lower:]]+ A string of lower case letters
\d+ A string of digits
# A literal #
It's not perfect - there're still many breakpoints. You can continue to refine these word definitions, but frankly, there's always going to be an edge case you can't catch. Then you wind up slowly expanding this regular expression until it's totally unmanageable. You could try using a dictionary, but that breaks down eventually, too. What do you do with "whirlwind"? Or "ITan"? Is that "IT an", or "I Tan"? Case in point? Here it is after I tried to catch some of My errors. It's getting so huge, and it's still trivial to come up with strings it breaks on. This function is all about degrees - how much time is it worth spending to teach your algorithm all the funny points of all the world languages?
EDIT: After some work, And deciding that I could be separated out as its own word if and only if it was followed immediately by One Capital letter and one lower case letter, I've updated my attempt at an answer.
function convert($keyword, $debug = false) {
$wResult = preg_match_all('/I(?=[[:upper:]][[:lower:]])|[[:upper:]]{2,}|[[:upper:]][[:lower:]]*|[[:lower:]]+|\d+|#/u', $keyword, $matches);
if($debug){
var_dump($matches);
var_dump($matches[0]);
var_dump(implode(' ',$matches[0]));
}
return implode(' ',$matches[0]);
}
I also added some new test cases:
convert("Icons") = "Icons"
convert("WellIKnowThat") == "Well I Know That"
convert("ITan") == "I Tan"
convert("whirlwind") == "whirlwind"
I think this is about as good as it's going to get today. The final set of "Word Definitions" in order of preference, is:
Upper case I, provided it's followed by an upper case letter and a lower case letter:I(?=[[:upper:]][[:lower:]])
Two or more consecutive upper case letters: [[:upper:]]{2,}
A single uppercase Letter, followed by as many Lower case letters as possible: [[:upper:]][[:lower:]]*
one or more consecutive lower case letters: [[:lower:]]+
One or more consecutive digits: \d+
A literal pound symbol: #
I've added another word definition, a test case, and refined the testing fiddle. The new word definition matches the rule for I, but with A - the only other one letter word in the English Language.
you need Unicode Regex:
\p{Lu} for upercase and \p{Li} for lowercase
Hence, your usage will look like this:
/([\p{Ll}0-9])?([\p{Lu}])/
my RegEx is written here and it does not work no matter how I change it, substitute characters what not. I have a list of strings that may have 3 words or 8 words. Is there a easier way to cut off the RegEx when we hit a certain character or string? Let me show you what I mean:
Here are some examples of strings I will deal with:
WKT8100 Cooperative Education Work Term Preparation 15 hrs/w
CST8259 Web Programming Languages II 5 hrs/w
CST8265 Web Security Basics 5 hrs/w
CST8267 Ecommerce 4 hrs/w
I want to extract only the course name and ID from the string and leave out the number of hours I need, so leaving me with:
WKT8100 Cooperative Education Work Term Preparation
as a return.
My RegEx currently is like this:
RegEx = "/[a-zA-Z]{3}[0-9]{4}[A-Z]{0,1}\s[a-zA-Z]{3,20}\s[a-zA-Z]{0,20}\s[a-zA-Z]{0,20}\s[a-zA-Z]{0,20}\s/";
I a RegEx that extracts the hours correctly so maybe if there is a method I can use with substr. That way I can basically extract everything before the hours RegEx and don't have to worry about a complex RegEx line.:
HoursRegEx = "#\s[0-9]{1,2}?\shrs\/w#i";
Why not:
/(.*) \d+ hrs\/w/
This should capture all characters before the x hrs/w part.
For a little more explanation, this just creates a capturing group that contains whatever it found before seeing: a space, one or more digits, another space, and then the sequence "hrs/w". Since you don't care what's before the end part, why try to recognize it?
If it always ends in " hrs/w", you can do this:
$string = "WKT8100 Cooperative Education Work Term Preparation 15 hrs/w";
$string = trim($string)
$lastSpace = strrpos($string, " ");
$string = trim(substr($string, 0, $lastSpace));
$lastSpace = strrpos($string, " ");
$hours = trim(substr($string, $lastSpace));
$nameID = trim(substr($string, 0, $lastSpace));
That's a way off the top of my head w/o using regex. I can't give you any regex without first doing some extensive refresher research.
p.s. Jordan's looks much cleaner.
I'm just getting to know regular expressions, but after doing quite a bit of reading (and learning quite a lot), I still have not been able to figure out a good solution to this problem.
Let me be clear, I understand that this particular problem might be better solved not using regular expressions, but for the sake of brevity let me just say that I need to use regular expressions (trust me, I know there are better ways to solve this).
Here's the problem. I'm given a big file, each line of which is exactly 4 characters long.
This is a regex that defines "valid" lines:
"/^[AB][CD][EF][GH]$/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";
}