I have the following regex:
^(?=.{8}$).+
The way I understand this is it will accept 8 of any type of character, followed by 1 or more of any character. I feel I am not grasping how a Positive Lookahead works. Because both sections of the Regex are looking for '.' wouldn't any series of characters fit this?
My question is, how does the positive lookahead effect this regex and what is an example of a matching string?
The following did not match when supplied in the following regex tool:
123456781
(12345678)1
(12345678)
(abcdefgh)a
(abcdefgh)
abc
123
EDIT: Removed first two data entries as I clearly wasn't using the regex tool correctly as they now match with exactly 8 characters.
^(?=.{8}$).+
will match the string
aaaaaaaa
Reasoning:
The content inside of the brackets is a lookahead, since it starts with ?=.
The content inside of a lookahead is parsed - it is not interpreted literally.
Thus, the lookahead only allows the regex to match if .{8}$ would match (at the start of the string, in this case). So the string has to be exactly eight characters then it has to end, as evidenced by $.
Then .+ will match those eight characters.
It is trying to match:
^ # start of line, but...
(?=.{8}$) # only if it precedes exactly 8 characters and the end of line
.+ # this one matches those 8 characters
and from your input, it should also match these (try this engine with match at line breaks checked):
12345678
abcdefgh
Matching 12345678 works in ruby:
'12345678' =~ /^(?=.{8}$).+/
=> 0
Maybe your test site don't support look ahead on regexps?
Related
I really don't use RegEx that much. You could say I am RegEx n00b. I have been working on this issue for a half a day.
I am trying to write a pattern that looks backward from a number character. For example:
1. bob1 => bob
2. cat3 => cat
3. Mary34 => Mary
So far I have this (?![A-Z][a-z]{1,})([A-Za-z_])
It only matches for individual characters, I want all the characters before the number character. I tried to add the ^ and $ into my pattern and using an online simulator. I am unsure where to put the ^ and $.
NOTE: I am using RegEx for the .NET Framework
You may use a regex like
[\p{L}_]+(?=\d)
or
[\w-[\d]]+(?=\d)
See the regex demo
Pattern details
[\p{L}_]+ - any 1 or more letters (both lower- and uppercase) and/or _
OR
[\w-[\d]]+ - 1 or more word chars except digits (the -[] inside a character class is a character class subtraction construct)
(?=\d) - a positive lookahead that requires a digit to appear immediately to the right of the current location
If we break down your RegEx, we see:
(?![A-Z][a-z]{1,}) which says "look ahead to find a string that is NOT one uppercase letter followed one or more lowercase letters" and ([A-Za-z_]) which says "match one letter or underscore". This should end up matching any single lowercase letter.
If I understand what you want to achieve, then you want all of the letters before a number. I would write something like that as:
\b([a-zA-Z]+)[0-9]
This will start at a word boundary \b, match one or more letters, and require a digit right after the matched string.
(The syntax I used seems to match this document about .NET RegEx: https://learn.microsoft.com/en-us/dotnet/standard/base-types/regular-expressions)
In light of Wiktor Stribizew's comment, here is a pure match RegEx:
\b[a-zA-Z_]+(?=[0-9])
This matches the pattern and then looks ahead for the digit. This is better than my first lookahead attempt. (Thank you Wiktor.)
http://www.rexegg.com/regex-lookarounds.html
I need to get a regex that will find a match of a single lower case a-z character followed by 5 numbers that is either:
at the start of a line
at the end of a line
surrounded by () or []
surrounded by whitespace
So the following results are expected:
a12345 MATCH
(a12345) MATCH
[a12345] MATCH
text a12345 MATCH
aa12345 NO MATCH
At the moment I have this (?<=[])]*)[a-z]{1}[0-9]{5}(?=[])]*) but it is not working for all scenarios, for example it sees aa12345 and a12345a as being matches when I don't want them to.
Can anyone help?
EDIT:
Apologies I should have mentioned this is for .NET c#
First of all your should mention programming language.
Following solution is for PCRE.
Regex: ((?<=[\[( ])|^)[a-z]\d{5}((?=[\]\) ])|$)
Explanation:
((?<=[\[( ])|^) checks for preceding brackets, whitespaces OR beginning.
[a-z]\d{5} checks for alphabet followed by 5 digits.
((?=[\]\) ])|$) checks for succeeding brackets, whitespaces OR end of line.
Regex101 Demo
Does this work:
(\[[a-z]\d{5}\])|(\([a-z]\d{5}\))|(\b[a-z]\d{5}\b)
I'm having trouble matching the start and end of a regex on Python.
Essentially I'm confused about the when to use word boundaries /b and start/end anchors ^ $
My regex of
^[A-Z]{2}\d{2}
matches 4 letter characters (two uppercase letters, two digits) which is what I'm after
Matches AJ99, RD22, CP44 etc
However, I also noted that AJAJAJAJAJAJAJAJAJSJHS99 could be matched as well. I've tried used ^ and $ together to match the whole string. This doesn't work
^[A-Z]{2}\d{2}$ # this doesn't work
but
^[A-Z]{2}\d{2} # this is fine
[A-Z]{2}\d{2}$ # this is fine
The string I'm matching against is 4 characters long, but in the first two examples the regex could pick the start and end of a longer string respectively.
s = "NZ43" # 4 characters, match perfect! However....
s = "AM27272727" # matches the first example
s = "HAHSHSHSHDS57" # matches the second example
The position anchors ^ and $ place a restriction on the position of your matched chars:
Analyzing your complete regex:
^[A-Z]{2}\d{2}$
^ matches only at the beginning of the text
[A-Z]{2} exactly 2 uppercase Ascii alphabetic characters
\d{2} exactly 2 digits (equivalent to [0-9]{2})
$ matches only at the end of the text
If you remove one or both of the 2 position anchors (^ or $) you can match a substring starting from the beginning or the end as you stated above.
If you want to match exactly a word without using the start/end of the string use the \b anchor, like this:
``\b[A-Z]{2}\d{2}\b``
\b matches at the start/end of text and between a regex word (in regex a word char \w is intended as one of [a-zA-Z0-9_]) and one char not in the word group (available as \W).
The regex above matches WS24 in all the next strings:
WS24 alone
before WS24
WS24 after
before WS24 after
NZ43
It doesn't match:
AM27272727 (it will do if is AM27 272727 or AM27"272727
HAHSHSHSHDS57 (it will do if HAHSHSHSH DS75 or...you get it)
A demo online (the site will be useful to you also to experiment with regex).
The fact that your shown behaviour is like it's supposed to be, your question suggests that you maybe does not have fully understood how regular expressions work.
As a addition to the very good and informative answer of GsusRecovery, here's a site, that guides you through the concepts of regular expressions and tries to teach you the basics with a lessons-based system. To be clear, I do not want to tout this website, as there are plenty of those, but however I could really made a use of this one and so it's the one I'm suggesting.
I'm trying to make a regex that matches a specific pattern, but I want to ignore lines starting with a #. How do I do it?
Let's say i have the pattern (?i)(^|\W)[a-z]($|\W)
It matches all lines with a single occurance of a letter. It matches these lines for instance:
asdf e asdf
j
kke o
Now I want to override this so that it does not match lines starting with a #
EDIT:
I was not specific enough. My real pattern is more complicated. It looks a bit like this: (?i)(^|\W)([a-hj-z]|lala|bwaaa|foo($|\W)
It should be used kind of like I want to block offensive language, if a line does not start with a hash, in which case it should override.
This is what you are looking for
^(?!#).+$
^ marks the beginning of line and $ marks the end of line(in multiline mode)
.+ would match 1 to many characters
(?!#) is a lookahead which would match further only if the line doesn't start with #
This regex will match any word character \w not preceeded by a #:
^(?<!#)\w+$
It performs a negative lookbehind at the start of the string and then follows it with 1 or more word characters.
I need to extract the last number that is inside a string. I'm trying to do this with regex and negative lookaheads, but it's not working. This is the regex that I have:
\d+(?!\d+)
And these are some strings, just to give you an idea, and what the regex should match:
ARRAY[123] matches 123
ARRAY[123].ITEM[4] matches 4
B:1000 matches 1000
B:1000.10 matches 10
And so on. The regex matches the numbers, but all of them. I don't get why the negative lookahead is not working. Any one care to explain?
Your regex \d+(?!\d+) says
match any number if it is not immediately followed by a number.
which is incorrect. A number is last if it is not followed (following it anywhere, not just immediately) by any other number.
When translated to regex we have:
(\d+)(?!.*\d)
Rubular Link
I took it this way: you need to make sure the match is close enough to the end of the string; close enough in the sense that only non-digits may intervene. What I suggest is the following:
/(\d+)\D*\z/
\z at the end means that that is the end of the string.
\D* before that means that an arbitrary number of non-digits can intervene between the match and the end of the string.
(\d+) is the matching part. It is in parenthesis so that you can pick it up, as was pointed out by Cameron.
You can use
.*(?:\D|^)(\d+)
to get the last number; this is because the matcher will gobble up all the characters with .*, then backtrack to the first non-digit character or the start of the string, then match the final group of digits.
Your negative lookahead isn't working because on the string "1 3", for example, the 1 is matched by the \d+, then the space matches the negative lookahead (since it's not a sequence of one or more digits). The 3 is never even looked at.
Note that your example regex doesn't have any groups in it, so I'm not sure how you were extracting the number.
I still had issues with managing the capture groups
(for example, if using Inline Modifiers (?imsxXU)).
This worked for my purposes -
.(?:\D|^)\d(\D)