给定一个包含电话号码列表（一行一个电话号码）的文本文件 file.txt，写一个 bash 脚本输出所有有效的电话号码。

你可以假设一个有效的电话号码必须满足以下两种格式： (xxx) xxx-xxxx 或 xxx-xxx-xxxx。（x 表示一个数字）

你也可以假设每行前后没有多余的空格字符。

示例：

假设file.txt有如下内容：

987-123-4567
123 456 7890
(123) 456-7890

你的脚本应该输出以下有效的电话号码:

987-123-4567
(123) 456-7890

有请Linux三剑客(awk,sed,grep)之一：

grep命令：

NAME
       grep, egrep, fgrep, rgrep - print lines matching a pattern

SYNOPSIS
       grep [OPTIONS] PATTERN [FILE...]
       grep [OPTIONS] -e PATTERN ... [FILE...]
       grep [OPTIONS] -f FILE ... [FILE...]

OPTIONS
       -n, --line-number
              Prefix each line of output with the 1-based line number within its
              input file.
       -r, --recursive
              Read  all  files  under  each  directory,  recursively,  following
              symbolic links only if they are on the command line.  Note that if
              no file operand is given, grep  searches  the  working  directory.
              This is equivalent to the -d recurse option.

REGULAR EXPRESSIONS
       The period . matches any single character.

   Character Classes and Bracket Expressions
       A bracket expression is a list of characters enclosed by [ and ].  It matches any single character in that list; if the first character of
       the list is the caret ^ then it matches any character not in the list.  For example,  the  regular  expression  [0123456789]  matches  any
       single digit.

       Within  a  bracket  expression, a range expression consists of two characters separated by a hyphen.  It matches any single character that
       sorts between the two characters, inclusive, using the locale's collating sequence and character set.   For  example,  in  the  default  C
       locale,  [a-d]  is  equivalent  to  [abcd].  Many locales sort characters in dictionary order, and in these locales [a-d] is typically not
       equivalent to [abcd]; it might be equivalent to [aBbCcDd], for example.  To obtain the traditional interpretation of bracket  expressions,
       you can use the C locale by setting the LC_ALL environment variable to the value C.

       Finally, certain named classes of characters are predefined within bracket expressions, as follows.  Their names are self explanatory, and
       they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:],  [:space:],  [:upper:],  and  [:xdigit:].
       For  example, [[:alnum:]] means the character class of numbers and letters in the current locale.  In the C locale and ASCII character set
       encoding, this is the same as [0-9A-Za-z].  (Note that the brackets in these class names are part of  the  symbolic  names,  and  must  be
       included  in  addition to the brackets delimiting the bracket expression.)  Most meta-characters lose their special meaning inside bracket
       expressions.  To include a literal ] place it first in the list.  Similarly, to include a literal ^ place it anywhere but first.  Finally,
       to include a literal - place it last.

   Anchoring
       The caret ^ and the dollar sign $ are meta-characters that respectively match the empty string at the beginning and end of a line.

   The Backslash Character and Special Expressions
       The  symbols  \< and \> respectively match the empty string at the beginning and end of a word.  The symbol \b matches the empty string at
       the edge of a word, and \B matches the empty string provided it's not at the edge of a word.  The symbol \w is a synonym for  [_[:alnum:]]
       and \W is a synonym for [^_[:alnum:]].

   Repetition
       A regular expression may be followed by one of several repetition operators:
       ?      The preceding item is optional and matched at most once.
       *      The preceding item will be matched zero or more times.
       +      The preceding item will be matched one or more times.
       {n}    The preceding item is matched exactly n times.
       {n,}   The preceding item is matched n or more times.
       {,m}   The preceding item is matched at most m times.  This is a GNU extension.
       {n,m}  The preceding item is matched at least n times, but not more than m times.

   Concatenation
       Two  regular  expressions  may be concatenated; the resulting regular expression matches any string formed by concatenating two substrings
       that respectively match the concatenated expressions.

   Alternation
       Two regular expressions may be joined by the infix operator |; the  resulting  regular  expression  matches  any  string  matching  either
       alternate expression.

   Precedence
       Repetition  takes  precedence  over concatenation, which in turn takes precedence over alternation.  A whole expression may be enclosed in
       parentheses to override these precedence rules and form a subexpression.

   Back References and Subexpressions
       The back-reference \n, where n is a single digit, matches the substring previously matched by the nth parenthesized subexpression  of  the
       regular expression.

   Basic vs Extended Regular Expressions
       In  basic regular expressions the meta-characters ?, +, {, |, (, and ) lose their special meaning; instead
       use the backslashed versions \?, \+, \{, \|, \(, and \).

常用用法展示：

$ grep -rn 12  
# -r表示在某个目录下递归搜索 默认路径是. 也就是当前目录
# -n搜索结果显示行号
file.txt:1:987-123-4567
file.txt:2:123 456 7890
file.txt:3:(123) 456-7890

本题解答

grep -P "^(\d{3}-|\(\d{3}\)\s)\d{3}-\d{4}$" file.txt

解析

-P后跟pattern字符串，注意到合法的电话号码尾部是一样的(xxx-xxxx)，只有开头有区别((xxx)空格或xxx-两种),所以pattern就是"^开头1|开头2尾部$"。数字可以用\d来表达，注意(和)要用\(和\)来转义的。