Sunday, February 04, 2007

Capturing vs. Non-capturing Regex Groups

I posted the following on Ben Nadel's excellent blog, in response to a question about why I use non-capturing groups in my regular expressions (e.g., (?:non-captured))...

I near-religiously use non-capturing groups whenever I do not need to reference a group's contents. There are only three reasons to use capturing groups:

  • You're using parts of a match to construct a replacement string, or otherwise referencing parts of the match in code outside the regex.
  • You need to reuse parts of the match within the regex itself. E.g., (["'])(?:\\\1|.)*?\1 would match values enclosed in either double or single quotes, while requiring that the same quote type start and end the match, and allowing inner, escaped quotes of the same type as the enclosure.
  • You need to test if an optional group was part of the match so far, as the condition to evaluate within a conditional. E.g., (a)?b(?(1)c|d) only matches the values "bd" and "abc".

There are two primary reasons to use non-capturing groups if a grouping doesn't meet one of the above conditions:

  • Capturing groups negatively impact performace, since creating backreferences requires that their contents be stored in memory. The performance hit may be tiny, especially when working with small strings, but it's there.
  • When you need to use several groupings in a single regex, only some of which you plan to reference later, it's very convenient to have the backreferences you want to use numbered sequentially. E.g., the logic in my parseUri() UDF could not be nearly as simple if I had not made appropriate use of capturing and non-capturing groups within the same regex.

On a related note, the values of backreferences created using capturing groups with repetition operators on the end of them may not be obvious until you're familar with how it works. If you ran the regex (.)* over the string "test", although backreference 0 (i.e., the whole match) would be "test", backreference 1 would be "t", and there would be no 2nd, 3rd, or 4th backreferences created for the strings "e," "s," and "t." If you wanted the entire match of a repeated grouping to be captured into a backreference, you could use, e.g., ((?:.)*). Also note that the way both of those patterns would be evaluated is fundamentally different from how regex engines would treat (.*).

8 comments:

Jose said...

Steve, thanks for the helpful post. I would just like to point out that when you say "there are only 3 reasons to use capturing groups" and right on the first reason you list an infinite set("use the captured regex outside the regex") you are in a bit of contradiction.

"Outside the regex" is a set made of a billion reasons.

I just thought I'd point that out.

Regards, Jose

Anonymous said...

TM产品还都支持网络广州翻译公司,报告昨日公布。比如,译员A刚刚翻译了韩语翻译共享记忆库功能。北京翻译公司也就是入深圳翻译公司说,当多人同时进行翻译时同声传译,可以通过局域网共享一个翻译记忆库"This is a file for demo.",当译员B遇到"This is a demo file."时,系统会给出A的译文"这是个演示用的文件。"翻译公司东莞翻译公司。在线翻译工具。法语翻译。B可以接受,也可以修改,修改后的译文又可供自己或他人重复使用。广州翻译公司,翻译记忆库就在这样的不断补充和完善过程中,发挥着越来越大的作同声传译设备租赁,是会议设备租赁,一项调查显示法语翻译几乎将深圳更多的是通过线翻译同声传译深圳俄语翻译
深圳韩语翻译广州同声传译用。
放大上海翻译公司这将导致人民币兑表决器出租,表决器销售 租赁表决器各种货币 德语翻译,,市场风险偏好升温。商务口译,料就在昨日下午稍晚时间,同传设备已经说明一切。翻译是一门严谨不容践踏的语言文化。同声传译,凡购深圳同声传译翻译部署促进房地产市场健康发展措施出台,深圳翻译.深圳英语翻译 ,无需制作炫丽的界面和复杂的操作功能深圳日语翻译,中国移动后台词库地产的阴霾情绪同声传译设备租赁,是会议设备租赁深圳手机号码,深圳手机靓号,有的用户同传设备出租会议同传系统租赁选择在线翻译会议设备租赁中美利差的一旦金融市场趋于稳定,。同声传译设备租赁存在,。新疆租车,美元汇率明年什么时候开始由强转弱, 广州翻译公司,用户的体验不能停留同声传译一扫而光”

Anonymous said...

ugg bootsTHE SNOW wholesale ugg bootsSTARTED TO FALLWholesale handbags SEVERAL HOURS wholesale clothingBEFORE HER laborWholesale jewelry began.wholesale clothing A few flakes wholesale handbagsfirst, wholesale furniturein the dull Furniture Wholesalegray late-afternoon Wholesale jewelrysky,Ceramic tileand then Micro sd cardswind-driven swirls Wholesale clothingand eddies Wholesale Jewelryaround the edges Wholesale fashion jewelryof their wide Wholesale costume jewelryfront porch. ugg bootsHe stood by her wholesaleside at the windowwholesale electronics, watching sharp gusts of snow billow,skin care products
then swirl and drift to the ground. All around the neighborhood, lights came on, and the naked branches of the trees turned white.

Anonymous said...

ugg bootsOutside,uggs snow continuedHigh pressure blower to fall quietly throughIndustrial fan the darkness, Industrial bloweras bright and thick Commercial bloweras static in the wholesalecones of light castchina wholesale by the streetlights.wholesale shoes By the time he rosewholesale clothing and looked out watchesthe window, pressure blowertheir car had become a soft white hill on the edge pressure blowersof the street.fans Already his footprints blowers
in the driveway had filled and disappeared.

Anonymous said...

wholesaleShe looked up, wholesale atvsmiling, wedding dresseswhen he tuckedMP3 player
the blanketwholesale mp3 around her legs.Wholesale Mp4
"You know,digital camera wholesale I've been wondering consumer electronicswhat it's like,"mp5 player she said. memory cards"Before we're born,Car video players I mean. It's too bad we can't remember." gps devicesShe opened her robe and pulled up the bluetooth headsetsweater she wore underneath, revealing a Mp3 Wholesalebelly as round and hard asMp3 Wholesale a melon. Mp4 WholesaleShe ran her hand across its smooth surface, Mp4 Wholesalefirelight playing across her skin, casting reddish gold onto her hair.

yiwu wholesale said...

AmandaIEC located in Yiwu, is a trusted one stop B2B export, trade, sourcing, wholesale site. You needn't to look for lots and lots Chinese small-and-medium-sized enterprises on B2B platform like alibaba.com, and then inquiry them one by one. You just send us an inquiry in detail, we do all other things. Browse Alphabetically: A,B,C,D,E,F,G,H,I,J,K,L,M,N,O,P,Q,R,S,T,U,V,W,X,Y,Z,0-9. Start your one stop China wholesale sourcing here today and experience first class service and fast shipping. Save your time, save your cost, and guard against fraud.

Anonymous said...

Making gw gold is the old question : Honestly there is no fast way to make lots of GuildWars Gold . Sadly enough a lot of the people that all of a sudden come to with millions of Guild Wars Gold almost overnight probably duped . Although there are a lot of ways to make lots of GuildWars moneyhere I will tell you all of the ways that I know and what I do to make cheap gw gold.

As a new player , you may need some game guides or information to enhance yourself.
habbo credits is one of the hardest theme for every class at the beginning . You must have a good way to manage yourhabbo gold.If yor are a lucky guy ,you can earn so many habbo coins by yourself . But if you are a not , I just find a nice way to get buy habbo gold. If you need , you can buycheap habbo credits at our website . Go to the related page and check the detailed information . Once you have any question , you can connect our customer service at any time .

Anonymous said...

網頁設計,情趣用品店,情趣用品專賣網

A片下載,成人影片下載
威而柔,自慰套,自慰套,SM,充氣娃娃,充氣娃娃,潤滑液,飛機杯,按摩棒,跳蛋,性感睡衣,威而柔,自慰套,自慰套,SM,充氣娃娃,充氣娃娃,潤滑液,飛機杯,按摩棒,跳蛋,性感睡衣
情惑用品性易購


免費視訊聊天室,aio交友愛情館,愛情公寓,一葉情貼圖片區,情色貼圖,情色文學,色情聊天室,情色小說,情色電影,情色論壇,成人論壇,辣妹視訊,視訊聊天室,情色視訊,免費視訊,免費視訊聊天,視訊交友網,視訊聊天室,視訊美女,視訊交友,視訊交友90739,AV,AV女優


A片,色情A片,免費A片,成人影片,色情影片,a片免費看,情色貼圖,情色文學,情色小說,色情小說


影音視訊聊天室