Alternation
If expression is a named or numbered capturing group, the alternation construct is interpreted as a capture test; for more information, see the next section, Conditional Matching Based on a Valid Capture Group. In other words, the regular expression engine does not attempt to match the captured substring, but instead tests for the presence or absence of the group.
alternation
If name does not correspond to the name of a capturing group that is used in the regular expression pattern, the alternation construct is interpreted as an expression test, as explained in the previous section. Typically, this means that expression evaluates to false. If number does not correspond to a numbered capturing group that is used in the regular expression pattern, the regular expression engine throws an ArgumentException.
The following example is a variation of the example that appears in the Either/Or Pattern Matching with section. It uses a capturing group named n2 that consists of two digits followed by a hyphen. The alternation construct tests whether this capturing group has been matched in the input string. If it has, the alternation construct attempts to match the last seven digits of a nine-digit U.S. Employer Identification Number (EIN). If it has not, it attempts to match a nine-digit U.S. Social Security Number (SSN).
When placed in a T-maze, rats or mice possess a strong tendency of alternating arm choices on successive trials. The exploration of novel environmental stimuli is dependent on the integrity of limbic and non-limbic pathways, including the basal forebrain, the hippocampus, the thalamus, the prefrontal cortex, and the dorsal striatum, as well as the vestibular system and cerebellum. Neurochemical pathways using acetylcholine, gamma-amino-butyric acid, and dopamine in the septum and hippocampus have been implicated in the exploration of novel maze arms. In addition to the delineation of interactions between neurotransmitters, the spontaneous alternation test is sensitive to the consequences of normal and pathological aging.
I already explained how you can use character classes to match a single character out of several possible characters. Alternation is similar. You can use alternation to match a single regular expression out of several possible regular expressions.
The alternation operator has the lowest precedence of all regex operators. That is, it tells the regex engine to match either everything to the left of the vertical bar, or everything to the right of the vertical bar. If you want to limit the reach of the alternation, you need to use parentheses for grouping. If we want to improve the first example to match whole words only, we would need to use \b(catdog)\b. This tells the regex engine to find a word boundary, then either cat or dog, and then another word boundary. If we had omitted the parentheses then the regex engine would have searched for a word boundary followed by cat, or, dog followed by a word boundary.
The regex engine starts at the first token in the regex, G, and at the first character in the string, S. The match fails. However, the regex engine studied the entire regular expression before starting. So it knows that this regular expression uses alternation, and that the entire regex has not failed yet. So it continues with the second option, being the second G in the regex. The match fails again. The next token is the first S in the regex. The match succeeds, and the engine continues with the next character in the string, as well as the next token in the regex. The next token in the regex is the e after the S that just successfully matched. e matches e. The next token, t matches t.
At this point, the third option in the alternation has been successfully matched. Because the regex engine is eager, it considers the entire alternation to have been successfully matched as soon as one of the options has. In this example, there are no other tokens in the regex outside the alternation, so the entire regex has successfully matched Set in SetValue.
The POSIX standard leaves it up to the implementation to choose a text-directed or regex-directed engine. A BRE that includes backreferences needs to be evaluated using a regex-directed engine. But a BRE without backreferences or an ERE can be evaluated using a text-directed engine. But the POSIX standard does mandate that the longest match be returned, even when a regex-directed engine is used. Such an engine cannot be eager. It has to continue trying all alternatives even after a match is found, in order to find the longest one. This can result in very poor performance when a regex contains multiple quantifiers or a combination of quantifiers and alternation, as all combinations have to be tried to find the longest match.
A series of studies was initiated to examine learning and memory function in the zebrafish (Danio rerio) by using a simple spatial alternation paradigm for a food reward. Fish were fed on alternating sides of a divided fish tank, with a red card displayed on one side serving as a visual means of orientation. Although responses were recorded at cue (light tap on the tank), 5 s after cue (as food was delivered), and 5 s after food delivery, the learning test was choice of a correct side of the tank to receive food. Therefore, an accurate level of an animal's achievement of the spatial task was represented by responses at food delivery. Data collected from 11 separate experiments indicated that zebrafish learned to alternate for a food reward. Further, statistical analysis showed that the zebrafish learned the task in the first half of the experiment as exhibited by a calculated t1/2 of 13.9 trials. Zebrafish could recall the task after a short period of 10 days with no testing. The alternating behavior was extinguished by withholding the food reward. Thus, the spatial alternation task can be learned easily by zebrafish, and may be useful in addressing learning and memory functions in vertebrate animals using zebrafish as a model organism.
Subjects are first placed in the start arm of the T Maze. Upon leaving the start arm, subjects choose between entering either the left or the right goal arm. With repeated trials, the animals should show less of a tendency to enter a previously visited arm. The percentage of alternation (number of turns in each goal arm) and total trial duration are recorded. This test is used to quantify cognitive deficits in transgenic strains of mice and evaluate novel chemical entities for their effects on cognition.
Testing occurs in a Y-shaped maze with three white, opaque plastic arms at a 120 angle from each other. After introduction to the center of the maze, the animal is allowed to freely explore the three arms. Over the course of multiple arm entries, the subject should show a tendency to enter a less recently visited arm. The number of arm entries and the number of triads are recorded in order to calculate the percentage of alternation. An entry occurs when all four limbs are within the arm. This test is used to quantify cognitive deficits in transgenic strains of mice and evaluate novel chemical entities for their effects on cognition.
This protocol details a method for using a T-maze to assess the cognitive ability of rodents. The T-maze is an elevated or enclosed apparatus in the form of a T placed horizontally. Animals are started from the base of the T and allowed to choose one of the goal arms abutting the other end of the stem. If two trials are given in quick succession, on the second trial the rodent tends to choose the arm not visited before, reflecting memory of the first choice. This is called 'spontaneous alternation'. This tendency can be reinforced by making the animal hungry and rewarding it with a preferred food if it alternates. Both spontaneous and rewarded alternation are very sensitive to dysfunction of the hippocampus, but other brain structures are also involved. Each trial should be completed in under 2 min, but the total number of trials required will vary according to statistical and scientific requirements.
Regular languages are closed under alternation, meaning that the alternation of two regular languages is again regular.[1] In implementations of regular expressions, alternation is often expressed with a vertical bar connecting the expressions for the two languages whose union is to be matched,[2][3] while in more theoretical studies the plus sign may instead be used for this purpose.[1] The ability to construct finite automata for unions of two regular languages that are themselves defined by finite automata is central to the equivalence between regular languages defined by automata and by regular expressions.[4]
Other classes of languages that are closed under alternation include context-free languages and recursive languages.The vertical bar notation for alternation is used in the SNOBOL language and some other languages.In formal language theory, alternation is commutative and associative. This is not in general true of the form of alternation used in pattern-matching languages, because of the side-effects of performing a match in those languages.
One challenge often faced by investigators when undertaking NCTs is reluctance by healthcare providers and organizations to use random allocation [6]. Thus, various alternatives to randomized designs have been developed and used to evaluate healthcare interventions [6, 7]. Historically, and prior to the advent of randomized trials in the 1940s, alternation was a common form of patient allocation to treatment comparison groups [8, 9]. The on-off design is an example of a NCT study design based on the concept of alternation, but which has not been widely described in the literature. The intent of this article is to describe the features of the on-off design, using two recent trials as examples. In addition, we explored the perceptions and experiences of healthcare providers tasked with implementing the on-off study design in their primary care settings. Therefore, our intent is to describe the on-off design, discussing potential strengths and weaknesses and its appropriateness to end users (i.e., healthcare providers and policy decisionmakers) of quality improvement research. 041b061a72