Repeat Regular Expression

Question

I need a regular expression that checks if every - in the string has a letter before and after it.

I got this so far:

(([-])?[a-zA-Z ]+[a-zA-Z]+[-]+[a-zA-Z]+[a-zA-Z ]+$|[a-zA-Z ]+$)|([a-zA-Z ])

works for these examples:

Tester
tester_test
Tes ter_te st

It doesn't work for these examples:

Tester_test tester_test
te st_te_st_te_st

That `regex` looks __much too__ complex for relatively simple matching. Consider simplifying it. — sshashank124, Apr 15 '14 at 12:16
Please could you elaborate what exactly you are trying to match? — Zlatin Zlatev, Apr 15 '14 at 12:18
What does it mean, "does not work"? how is it failing? what do you want to do? — Robin, Apr 15 '14 at 12:18
Every - in the string needs to have a letter before and after it. — Lendl Verschoor, Apr 15 '14 at 12:36
Don't know what you mean by flavour for regex. I am using it for a ASP.NET element. — Lendl Verschoor, Apr 15 '14 at 12:45
You can check for matches of `[^a-Z][-][^a-Z]` and if found, you have the negative result. — hjpotter92, Apr 15 '14 at 12:45
What exactly is your purpose here? To capture words that contain underscores that are *not* surrounded by letters, or that *only* have "correct" underscores in them? Some context of what your actually doing this for would be very helpful. — aliteralmind, Apr 15 '14 at 12:55

score 1 · Accepted Answer · edited May 23 '17 at 12:27

1

To ensure that every underscore is surrounded by a letter, you can use

(?<=[a-zA-Z])_(?=[a-zA-Z])

Regular expression visualization

Debuggex Demo

This demo works for all of your inputs.

This uses a positive lookbehind ((?<=...)) and lookahead ((?=...)).

Please consider bookmarking the Stack Overflow Regular Expressions FAQ for future reference. There is a section on lookarounds that may be of interest, as well as a list of online testers at the bottom.

edited May 23 '17 at 12:27

Community

1
1

answered Apr 15 '14 at 12:47

aliteralmind

19,847
17
77
108

What does ?<= mean? I don't find anything about this in google. – Xatenev Apr 15 '14 at 12:53
@Xatenev: Updated my answer. They're the lookaheads and behinds I mentioned. – aliteralmind Apr 15 '14 at 12:56
Uncaught SyntaxError: Invalid regular expression – Lendl Verschoor Apr 15 '14 at 13:32
1

This is structurally one of the best regex related answers I have seen in a while! Sadly debuggex only supports javascript, python and perl. The ASP.NET syntax might be different and the lookahead feature might not even exist there - I dont know. @eXo the mentioned FAQ is really helpful, go check it out. – SebastianH Apr 15 '14 at 14:01
It works for me at regexhero, which is an online tester for .NET regular expressions: http://regexhero.net/tester/?id=35632c5b-ccbd-430b-a8e5-97f833b1b08f. @eXo: Are you going to give us some more information so we can help you better, or are you just going to throw error-messages at us? – aliteralmind Apr 15 '14 at 14:06

score 1 · Answer 2 · answered Apr 15 '14 at 13:13

1

This regex seemed to work for all your Examples, and aswell as alot of others. Check for the negation of following:

^(.*(([^a-zA-Z]-)|(-[^a-zA-Z])).*)$

Check this regex out here, with a lot of examples:

http://www.rubular.com/r/o2OZYJheIt

answered Apr 15 '14 at 13:13

Xatenev

6,383
3
18
42

I thought the requirements were that a letter needs to be on *both* sides of the underscore/dash. Meaning `a-a` is okay, and `a--a`, `a-`, and `$-` are not. – aliteralmind Apr 15 '14 at 13:31
I dont think ruby is used by the questioner. – SebastianH Apr 15 '14 at 14:03
@SebastianH the Regex is quite similiar to his probably. Regex doesn't change thaat much. – Xatenev Apr 15 '14 at 14:37

Repeat Regular Expression

2 Answers2