How to validate whether a regex itself is valid or not?
Is there a regular expression to detect a valid regular expression?
How to apply multiple styles in WPF In WPF, how would I apply multiple styles to a FrameworkElement?  For instance, I have a control which already has a style.  I also have a separate style which I would like to add to it without blowing away the first one.  The styles have different TargetTypes, so I can't just extend one with the other. A not free $\mathbb{Z}$-module From  I see that $\mathbb{Z}^{\mathbb{N}}$ is not a free abelian group. Someone can explain me why?  Thanks a lot! Boosts are non-unitary! Unlike rotations, the boost transformations are non-unitary. Therefore, the boost generators are not Hermitian. When boosts induce transformations in the Hilbert space, will those transformation be unitary? I think no. If that is the case, what is the physical significance of such non-unitary transformations corresponding to boosts in the Hilbert space? What does the colon in Tikz coordinates mean? Absolute zero and Heisenberg uncertainty principle I got to read Volume I of Feynmann's lectures. It said that at absolute zero, molecular motion doesn't cease at all, because if that happens, we will be able to make precise determination of position and momentum of the atom. We do know that Heisenberg uncertainty principle holds for microscopic particles in motion. But what then is wrong to consider that all molecular motion ceases at absolute zero? In other words, does the uncertainty principle not hold when there is no motion?  Need some help! Android Respond To URL in Intent I want my intent to be launched when the user goes to a certain url: for example, the android market does this with  urls. so does youtube. I want mine to do that too. At lunch, my coworkers and I were discussing The Force Awakens, and started questioning Chewbacca's age.  A quick check of  and it says he is possibly about 230 years old as of Episode VII.  Following the  link on that page, Wookieepedia claims that they live to be 400 years old.  Do we know from canon what the average life expectancy of a Wookiee is? Why don't the White Walkers take the sea route to Westeros Is there a word for people who always act too friendly in order to curry favor with their acquaintances? Is there a single noun or adjective for a person who is always flattering friends and acquaintances, not only to be liked and accepted but, at other times, to curry favor with them as well ?  I'm not looking for regionalisms but a word that will be understood in any English speaking country. Is it possible to add new alphabets to unicode-math? How to split numbers into columns with negative and positive values, without empty cells? How to create an interactive bash script to collect arguments for a command in linux? I'm trying to create an interactive bash script where I can collect user input variables to be used as arguments in another command. Can someone provide me an example of this?  Currently i want to do:   what type of content you want to delete? what is the status?   then feed it into a mysql delete query command. I was looking for some shell commands, and I took a look in "users" command.  I was hoping to get only 1 name, but I got 2:  nori@nori-hidamari:~$ users nori nori   And when I run the command "who"  nori@nori-hidamari:~$ who nori     :0           2015-04-09 09:31 (:0) nori     pts/0        2015-04-09 09:48 (:0)   Is this behaviour normal? Or it's bad?  Thank you guys for helping me. Activate Field Service Lightning feature in a Salesforce DX scratch org? I am struggling to figure out how to -- or if it's even possible to -- create a DX scratch org with the "Field Service Lightning" (FSL) feature installed. Based on the feature list  I'm guessing the answer is no and I'll just have to wait. But I'm not sure I'm interpreting that source correctly or missing something entirely since I'm just wrapping my head around DX.   Background: we have a production org with FSL installed and active, along with it's accompanying managed packages. I can create sandboxes from it and do development using standard Metadata API tools, but I was hoping to start a new project using DX. But I'm stuck configuring a scratch org with the proper features. Prove by induction that... $1+3+5+7+...+(2n+1)=(n+1)^2$ for every $n \in \mathbb N$ NewForm.aspx disappeared I'm looking for an elegant way to show that, among non-negative numbers, $$ \max \{a_1 + b_1, \dots, a_n + b_n\} \leq \max \{a_1, \dots, a_n\} + \max \{b_1, \dots, b_n\} $$  I can show that $\max \{a+b, c+d\} \leq \max \{a,c\} + \max \{b,d\}$ by exhaustively checking all possibilities of orderings among $a,c$ and $b,d$.  But, I feel like there should be a more intuitive/efficient way to show this property for arbitrary sums like the one above. What happens if 2 separate abilities require you to sacrifice a creature? Will Area 51 be supported in the app? There are many (duplicate) questions about the acceptance, popularity and history of singular they (and their, them and themself) around here, it even got a tag of its own. If I didn’t miss anything, however, the proper verb form hasn’t been questioned yet.  As we all know, English third person singular pronouns (it, she, he and one, +body), names (Alice, Bob, …) and nouns (student, teacher, …) demand the +(e)s suffix be added to the finite verb form in simple present, where some auxiliary verbs have a “special” form (is &lt; *bes / *ares and has &lt; *haves). All other subjects don’t, including plural third person pronoun they.  When the plural you replaced thou (with thee, thy / thine), the other marked verb form that had remained in English – i.e. suffix +(e)st or +t – vanished, too. The first and second persons only ever appear as pronouns (I, we; you), not names or nouns, so there was no strong inclination to keep the verbal inflection.  The second person precedent would suggest that singular they be used with uninflected verb forms which is how it’s usually encountered in the wild. Assuming that they someday replaces he and she (and maybe it) it would lead to disagreement with the words the pronoun stands in for:        Alice         goes to her   place  by herself.   Bob           goes to his   place  by himself.   Alice and Bob go   to their place  by themselves. – (common)   Alice and Bob go   to their places by themselves. – (separate)                  She  goes to her   place  by herself.   He   goes to his   place  by himself.   They go   to their place  by themselves.   They go   to their places by themselves.                  They ?goes to their place  by themself.   ditto   They  go   to their place  by themselves.   They  go   to their places by themselves.                  They ?go to their place  by ?themselves.   ditto   They  go to their place  by  themselves.   They  go to their places by  themselves.      So why doesn’t singular they afford s on present-tense verbs like all other third person singular subjects do?   I learned about singular they only long after I had been taught English as a second language in school. That’s why it’s still a conscious decision to use it and hence I could easily adapt to use s forms with it, but would that sound and look funny / strange / wrong to native speakers?