A period p of a string w is any positive integer p such that w[i]=w[i+p]
whenever both sides of this equation are defined. Let per(w) denote
the size of the smallest period of w . We say that a string w is
periodic iff per(w) <= |w|/2.
So informally a periodic string is just a string that is made up from a prefix repeated at least twice. The only complication is that at the end of the string we don't require a full copy of the prefix.
For, example consider the string x = abcab. per(abcab) = 3 as x[1] = x[1+3] = a, x[2]=x[2+3] = b and there is no smaller period. The string abcab is therefore not periodic. However, the string ababa is periodic as per(ababa) = 2.
As more examples, abcabca, ababababa and abcabcabc are also periodic.
Is there a regex to determine if a string is periodic or not?
I don't really mind which flavor of regex but if it makes a difference, anything that Python re supports.
What you need is backreference
\b(\w*)(\w+\1)\2+\b
This matches even abcabca and ababababa.
\1 and \2 are used to match the first and second capturing groups, respectively.
You could use Regex back references.
For example (.+)\1+. This pattern will match a group () formed of at least one character .+. This group \1 (back reference) must repeat at least one time for a match.
The string ababa matches and it finds ab as the 1st group.
The string abcab is not a match.
Later edit
If you want a prefix that is repeated at least twice, you can change the pattern to: ^(.+)\1+. The problem is that I don't think you can match the end of the string to a substring of the prefix. So any string that starts with a repeating pattern will match but it will ignore the ending of the string.
Even later edit
Inspired from #tobias_k answer, here is how I would do it ^((.+)(?:.*))\1+\2?$. It looks for a string that has a prefix (it looks for the longest prefix it can find) that repeats at least twice and the ending must be the starting part of the prefix.
The first capturing group from the match will be the prefix that is repeating.
https://regex101.com/r/jQ3yY1/2
If you want the shortest prefix that repeats, you can use this pattern ^((.+?)(?:.*?))\1+\2?$.
You can use a regex like ^(.+)(.*)(\1\2)+\1?$.
^...$ from start to end of string
(.+) part of period that is always repeated (e.g. a in ababa)
(.*) optional part of period that is repeated except at the end (e.g. b in ababa)
(\1\2)+ one or more repetitions of the entire period
\1? optional final repetition of first part of the period
In Python:
>>> p = r"^(.+)(.*)(\1\2)+\1?$"
>>> re.match(p, "abcab")
None
>>> re.match(p, "abcabca")
<_sre.SRE_Match at 0x7f5fde6e51f8>
Note that this does not match the empty string "" though, which could also be considered periodic. If the empty string should be matched, you will have to treat it separately, e.g. by simply appending |^$ at the end of the regex.
Related
I need to write a regular expression that has to replace everything except for a single group.
E.g
IN
OUT
OK THT PHP This is it 06222021
This is it
NO MTM PYT Get this content 111111
Get this content
I wrote the following Regular Expression: (\w{0,2}\s\w{0,3}\s\w{0,3}\s)(.*?)(\s\d{6}(\s|))
This RegEx creates 4 groups, using the first entry as an example the groups are:
OK THT PHP
This is it
06222021
Space Charachter
I need a way to:
Replace Group 1,2,4 with String.Empty
OR
Get Group 3, ONLY
You don't need 4 groups, you can use a single group 1 to be in the replacement and match 6-8 digits for the last part instead of only 6.
Note that this \w{0,2} will also match an empty string, you can use \w{1,2} if there has to be at least a single word char.
^\w{0,2}\s\w{0,3}\s\w{0,3}\s(.*?)\s\d{6,8}\s?$
^ Start of string
\w{0,2}\s\w{0,3}\s\w{0,3}\s Match 3 times word characters with a quantifier and a whitespace in between
(.*?) Capture group 1 match any char as least as possible
\s\d{6,8} Match a whitespace char and 6-8 digits
\s? Match an optional whitespace char
$ End of string
Regex demo
Example code
Dim s As String = "OK THT PHP This is it 06222021"
Dim result As String = Regex.Replace(s, "^\w{0,2}\s\w{0,3}\s\w{0,3}\s(.*?)\s\d{6,8}\s?$", "$1")
Console.WriteLine(result)
Output
This is it
My approach does not work with groups and does use a Replace operation. The match itself yields the desired result.
It uses look-around expressions. To find a pattern between two other patterns, you can use the general form
(?<=prefix)find(?=suffix)
This will only return find as match, excluding prefix and suffix.
If we insert your expressions, we get
(?<=\w{0,2}\s\w{0,3}\s\w{0,3}\s).*?(?=\s\d{6}\s?)
where I simplified (\s|) as \s?. We can also drop it completely, since we don't care about trailing spaces.
(?<=\w{0,2}\s\w{0,3}\s\w{0,3}\s).*?(?=\s\d{6})
Note that this works also if we have more than 6 digits because regex stops searching after it has found 6 digits and doesn't care about what follows.
This also gives a match if other things precede our pattern like in 123 OK THT PHP This is it 06222021. We can exclude such results by specifying that the search must start at the beginning of the string with ^.
If the exact length of the words and numbers does not matter, we simply write
(?<=^\w+\s\w+\s\w+\s).*?(?=\s\d+)
If the find part can contain numbers, we must specify that we want to match until the end of the line with $ (and include a possible space again).
(?<=^\w+\s\w+\s\w+\s).*?(?=\s\d+\s?$)
Finally, we use a quantifier for the 3 ocurrences of word-space:
(?<=^(\w+\s){3}).*?(?=\s\d+\s?$)
This is compact and will only return This is it or Get this content.
string result = Regex.Match(#"(?<=^(\w+\s){3}).*?(?=\s\d+\s?$)").Value;
I do not understand how regex string matching works
r2 = r'a[bcd]*b'
m1 = re.findall(r2,"abcbd")
abcb
This falls in line with what was explained in regex
Step 3 The engine tries to match b, but the current position is at the end of the string, so it fails.
How?I do not understand this?
The following regex a[bcd]*b matches the longest substring (because * is greedy):
a starting with a
[bcd]* followed by any number (0: can match empty string) of character in set (b,c,d)
b ending by b
EDIT: following comment, backtracking occurs in following example
>>> re.findall(r2,"abcxb")
['ab']
abc matches a[bcd]*, but x is not expected
a also matches a[bcd]* (because empty string matches [bcd]*)
finally returns ab
Concerning greediness, the metacharacter * after a single character, a character set or a group, means any number of times (the most possible match) some regexp engines accept the sequence of metacharacters *? which modifies the behavior to the least possible, for example:
>>> r2 = r'a[bcd]*?b'
>>> re.findall(r2,"abcbde")
['ab']
Your regular expression requires the match to end in b, therefore everything is matched up to the trailing d. If b were optional, as in a[bcd]*b?, then entire string would be matched.
I have a command-line program that its first argument ( = argv[ 1 ] ) is a regex pattern.
./program 's/one-or-more/anything/gi/digit-digit'
So I need a regex to check if the entered input from user is correct or not. This regex can be solve easily but since I use c++ library and std::regex_match and this function by default puts begin and end assertion (^ and $) at the given string, so the nan-greedy quantifier is ignored.
Let me clarify the subject. If I want to match /anything/ then I can use /.*?/ but std::regex_match considers this pattern as ^/.*?/$ and therefore if the user enters: /anything/anything/anyhting/ the std::regex_match still returns true whereas the input-pattern is not correct. The std::regex_match only returns true or false and the expected pattern form the user can only be a text according to the pattern. Since the pattern is various, here, I can not provide you all possibilities, but I give you some example.
Should be match
/.//
s/.//
/.//g
/.//i
/././gi
/one-or-more/anything/
/one-or-more/anything/g/3
/one-or-more/anything/i
/one-or-more/anything/gi/99
s/one-or-more/anything/g/4
s/one-or-more/anything/i
s/one-or-more/anything/gi/54
and anything look like this pattern
Rules:
delimiters are /|##
s letter at the beginning and g, i and 2 digits at the end are optional
std::regex_match function returns true if the entire target character sequence can be match, otherwise return false
between first and second delimiter can be one-or-more +
between second and third delimiter can be zero-or-more *
between third and fourth can be g or i
At least 3 delimiter should be match /.// not less so /./ should not be match
ECMAScript 262 is allowed for the pattern
NOTE
May you would need to see may question about std::regex_match:
std::regex_match and lazy quantifier with strange
behavior
I no need any C++ code, I just need a pattern.
Do not try d?([/|##]).+?\1.*?\1[gi]?[gi]?\1?d?\d?\d?. It fails.
My attempt so far: ^(?!s?([/|##]).+?\1.*?\1.*?\1)s?([/|##]).+?\2.*?\2[gi]?[gi]?\d?\d?$
If you are willing to try, you should put ^ and $ around your pattern
If you need more details please comment me, and I will update the question.
Thanks.
You could use this regular expression:
^s?([/|##])((?!\1).)+\1((?!\1).)*\1((gi?|ig)(\1\d\d?)?|i)?$
See regex101.com
Note how this also rejects these cases:
///anything/
/./anything/gg
/./anything/ii
/./anything/i/12
How it works:
Some explanation of the parts that are different:
((?!\1).): this will match any character that is not the delimiter. This way you are sure you can keep track of the exact number of delimiters used. You can this way also prevent that the first character after the first delimiter, is again that delimiter, which should not be allowed.
(gi?|ig): matches any of the valid modifier combinations, except a sole i, which is treated separately. So this also excludes gg and ii as valid character sequences.
(\1\d\d?)?: optionally allows for an extra delimiter (after a g modifier -- see previous) to be added with one or two digits following it.
( |i)?: for the case there is no g modifier present, but just the i or none: then no digits are allowed to follow.
This is a tricky one, but I took the challenge - here is what I have ended up with:
^s?([\/|##])(?:(?!\1).)+\1(?:(?!\1).)*\1(?:i|(?:gi?|ig)(\1\d{1,2})?)?$
Pattern breakdown:
^ matches start of string
s? matches an optional 's' character
([\/|##]) matches the delimeter characters and captures as group 1
(?:(?!\1).)+ matches anything other than the delimiter character one or more times (uses negative lookahead to make sure that the character isn't the delimiter matched in group 1)
\1 matches the delimiter character captured in group 1
(?:(?!\1).)* matches anything other than the delimiter character zero or more times
\1 matches the delimiter character captured in group 1
(?: starts a new group
i matches the i character
| or
(?:gi?|ig) matches either g, gi, or ig
(\1\d{1,2})? followed by an optional extra delimiter and 0-9 once or twice
)? closes group and makes it optional
$ matches end of string
I have used non capturing groups throughout - these are groups that start ?:
This is a crossword problem. Example:
the solution is a 6-letter word which starts with "r" and ends with "r"
thus the pattern is "r....r"
the unknown 4 letters must be drawn from the pool of letters "a", "e", "i" and "p"
each letter must be used exactly once
we have a large list of candidate 6-letter words
Solutions: "rapier" or "repair".
Filtering for the pattern "r....r" is trivial, but finding words which also have [aeip] in the "unknown" slots is beyond me.
Is this problem amenable to a regex, or must it be done by exhaustive methods?
Try this:
r(?:(?!\1)a()|(?!\2)e()|(?!\3)i()|(?!\4)p()){4}r
...or more readably:
r
(?:
(?!\1) a () |
(?!\2) e () |
(?!\3) i () |
(?!\4) p ()
){4}
r
The empty groups serve as check marks, ticking off each letter as it's consumed. For example, if the word to be matched is repair, the e will be the first letter matched by this construct. If the regex tries to match another e later on, that alternative won't match it. The negative lookahead (?!\2) will fail because group #2 has participated in the match, and never mind that it didn't consume anything.
What's really cool is that it works just as well on strings that contain duplicate letters. Take your redeem example:
r
(?:
(?!\1) e () |
(?!\2) e () |
(?!\3) e () |
(?!\4) d ()
){4}
m
After the first e is consumed, the first alternative is effectively disabled, so the second alternative takes it instead. And so on...
Unfortunately, this technique doesn't work in all regex flavors. For one thing, they don't all treat empty/failed group captures the same. The ECMAScript spec explicitly states that references to non-participating groups should always succeed.
The regex flavor also has to support forward references--that is, backreferences that appear before the groups they refer to in the regex. (ref) It should work in .NET, Java, Perl, PCRE and Ruby, that I know of.
Assuming that you meant that the unknown letters must be among [aeip], then a suitable regex could be:
/r[aeip]{4,4}r/
What's the front end language being used to compare strings, is it java, .net ...
here is an example/psuedo code using java
String mandateLetters = "aeio"
String regPattern = "\\br["+mandateLetters+"]*r$"; // or if for specific length \\br[+mandateLetters+]{4}r$
Pattern pattern = Pattern.compile(regPattern);
Matcher matcher = pattern.matcher("is this repair ");
matcher.find();
Why not replace each '.' in your original pattern with '[aeip]'?
You'd wind up with a regex string r[aeip][aeip][aeip][aeip]r.
This could of course be shortened to r[aeip]{4,4}r, but that would be a pain to implement in the general case and probably wouldn't improve the code any.
This doesn't address the issue of duplicate letter use. If I were coding it, I'd handle that in code outside the regexp - mostly because the regexp would get more complicated than I'd care to handle.
So the "only once" part is the critical thing. Listing all permutations is obviously not feasible. If your language/environment supports lookaheads and backreferences you can make it a bit easier for yourself:
r(?=[aeip]{4,4})(.)(?!\1)(.)(?!\1|\2)(.)(?!\1|\2|\3).r
Still quite ugly, but here is how it works:
r # match an r
(?= # positive lookahead (doesn't advance position of "cursor" in input string)
[aeip]{4,4}
) # make sure that there are the four desired character ahead
(.) # match any character and capture it in group 1
(?!\1)# make sure that the next character is NOT the same as the previous one
(.) # match any character and capture it in group 2
(?!\1|\2)
# make sure that the next character is neither the first nor the second
(.) # match any character and capture it in group 3
(?!\1|\2|\3)
# same thing again for all three characters
. # match another arbitrary character
r # match an r
Working demo.
This is neither really elegant nor scalable. So you might just want to use r([aiep]{4,4})r (capturing the four critical letters) and ensure the additional condition without regex.
EDIT: In fact, the above pattern is only really useful and necessary if you just want to ensure that you have 4 non-identical characters. For your specific case, again using lookaheads, there is simpler (despite longer) solution:
r(?=[^a]*a[^a]*r)(?=[^e]*e[^e]*r)(?=[^i]*i[^i]*r)(?=[^p]*p[^p]*r)[aeip]{4,4}r
Explained:
r # match an r
(?= # lookahead: ensure that there is exactly one a until the next r
[^a]* # match an arbitrary amount of non-a characters
a # match one a
[^a]* # match an arbitrary amount of non-a characters
r # match the final r
) # end of lookahead
(?=[^e]*e[^e]*r) # ensure that there is exactly one e until the next r
(?=[^i]*i[^i]*r) # ensure that there is exactly one i until the next r
(?=[^p]*p[^p]*r) # ensure that there is exactly one p until the next r
[aeip]{4,4}r # actually match the rest to include it in the result
Working demo.
For r....m with a pool of deee, this could be adjusted as:
r(?=[^d]*d[^d]*m)(?=[^e]*(?:e[^e])*{3,3}m)[de]{4,4}m
This ensures that there is exactly one d and exactly 3 es.
Working demo.
not fully regex due to sed multi regex action
sed -n -e '/^r[aiep]\{4,\}r$/{/\([aiep]\).*\1/!p;}' YourFile
take pattern 4 letter in group aeipsourround by r, keep only line where no letter in the sub group is found twice.
A more scalable solution (no need to write \1, \2, \3 and so on for each letter or position) is to use negative lookahead to assert that each character is not occurring later:
^r(?:([aeip])(?!.*\1)){4}r$
more readable as:
^r
(?:
([aeip])
(?!.*\1)
){4}
r$
Improvements
This was a quick solution which works in the situation you gave us, but here are some additional constraints to have a robuster version:
If the "pool of letters" may share some letters with the end of string, include the end of pattern in the lookahead:
^r(?:([aeip])(?!.*\1.*\2)){4}(r$)
(may not work as intended in all regex flavors, in which case copy-paste the end of pattern instead of using \2)
If some letters must be present not only once but a different fixed number of times, add a separate lookahead for all letters sharing this number of times. For instance, "r....r" with one "a" and one "p" but two "e" would be matched by this regex (but "rapper" and "repeer" wouldn't):
^r(?:([ap])(?!.*\1.*\3)|([e])(?!.*\2.*\2.*\3)){4}(r$)
The non-capturing groups now has 2 alternatives: ([ap])(?!.*\1.*\3) which matches "a" or "p" not followed anywhere until ending by another one, and ([e])(?!.*\2.*\2.*\3) which matches "e" not followed anywhere until ending by 2 other ones (so it fails on the first one if there are 3 in total).
BTW this solution includes the above one, but the end of pattern is here shifted to \3 (also, cf. note about flavors).
I have a string like:
$str1 = "12 ounces";
$str2 = "1.5 ounces chopped;
I'd like to get the amount from the string whether it is a decimal or not (12 or 1.5), and then grab the immediately preceding measurement (ounces).
I was able to use a pretty rudimentary regex to grab the measurement, but getting the decimal/integer has been giving me problems.
Thanks for your help!
If you just want to grab the data, you can just use a loose regex:
([\d.]+)\s+(\S+)
([\d.]+): [\d.]+ will match a sequence of strictly digits and . (it means 4.5.6 or .... will match, but those cases are not common, and this is just for grabbing data), and the parentheses signify that we will capture the matched text. The . here is inside character class [], so no need for escaping.
Followed by arbitrary spaces \s+ and maximum sequence (due to greedy quantifier) of non-space character \S+ (non-space really is non-space: it will match almost everything in Unicode, except for space, tab, new line, carriage return characters).
You can get the number in the first capturing group, and the unit in the 2nd capturing group.
You can be a bit stricter on the number:
(\d+(?:\.\d*)?|\.\d+)\s+(\S+)
The only change is (\d+(?:\.\d*)?|\.\d+), so I will only explain this part. This is a bit stricter, but whether stricter is better depending on the input domain and your requirement. It will match integer 34, number with decimal part 3.40000 and allow .5 and 34. cases to pass. It will reject number with excessive ., or only contain a .. The | acts as OR which separate 2 different pattern: \.\d+ and \d+(?:\.\d*)?.
\d+(?:\.\d*)?: This will match and (implicitly) assert at least one digit in integer part, followed by optional . (which needs to be escaped with \ since . means any character) and fractional part (which can be 0 or more digits). The optionality is indicated by ? at the end. () can be used for grouping and capturing - but if capturing is not needed, then (?:) can be used to disable capturing (save memory).
\.\d+: This will match for the case such as .78. It matches . followed by at least one (signified by +) digit.
This is not a good solution if you want to make sure you get something meaningful out of the input string. You need to define all expected units before you can write a regex that only captures valid data.
use this regular expression \b\d+([\.,]\d+)?
To get integers and decimals that either use a comma or a dot plus the next word, use the following regex:
/\d+([\.,]\d+)?\s\S+/