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A completely different set of special characters existsinside character sets: ` ]', ` -' and ` ^'. Note that the usual special characters are not special any more inside acharacter set. Ranges may be intermixed freely with individualcharacters, as in ` ', which matches any lower case letter or` $', ` %' or period. ` ' ` '.In the simplest case, the characters between the two form the set.Thus, ` ' matches either ` a' or ` d',and ` *' matches any string of ` a''s and ` d''s(including the empty string), from which it follows that` c*r' matches ` car', et cetera.Ĭharacter ranges can also be included in a character set, by writing twocharacters with a ` -' between them. Thus, ` c?r' matches ` cr' or ` car'or ` cdr', and nothing else. ` ?' ` ?' is like ` *' except that it allows either zero or one match forthe preceding pattern. Thus, ` c+r' does not match` cr' but does match anything else that ` c*r' would match.
![regular expression not a number regular expression not a number](https://boulderinsight.com/wp-content/uploads/2019/09/reg_ex.jpg)
` +' ` +' is like ` *' except that at least one match for the precedingpattern is required for ` +'. Thus, ` fo*' has a repeating ` o', not a repeating` fo'. ` *' always applies to the smallest possible precedingexpression. The case of zero ` o''s is allowed: ` fo*' does match` f'. In` fo*', the ` *' applies to the ` o', so ` fo*' matches` f' followed by any number of ` o''s. ` *' is not a construct by itself it is a suffix, which means the precedingregular expression is to be repeated as many times as possible. Usingconcatenation, we can make regular expressions like ` a.b' which matchesany three-character string which begins with ` a' and ends with` b'. ' is a special character that matches anything except a newline. Any character not mentioned here is notspecial it stands for exactly itself for the purposes of searching andmatching. The following are the characters and character sequences which have specialmeaning within regular expressions. It is poor practice to depend onthis behavior better to quote the special character anyway, regardless ofwhere is appears. For example,` *foo' treats ` *' as ordinary since there is no precedingexpression on which the ` *' can act. Note: special characters are treated as ordinary ones if they are incontexts where their special meanings make no sense. (It does not match the string ` ff'.) Likewise,` o' is a regular expression that matches only ` o'.Īny two regular expressions a and b can be concatenated.The result is a regular expression which matches a string if amatches some amount of the beginning of that string and bmatches the rest of the string.Īs a simple example, we can concatenate the regular expressions` f' and ` o' to get the regular expression ` fo',which matches only the string ` fo'. Any othercharacter appearing in a regular expression is ordinary, unless a ` \'precedes it.įor example, ` f' is not a special character, so it is ordinary,and therefore ` f' is a regular expression that matches the string ` f'and no other string. An ordinary character is asimple regular expression which matches that character and nothing else. Regular expressions have a syntax in which a few characters are specialconstructs and the rest are ordinary. For example, the regular expression` foo|bar' matches either the string ` foo' or the string` bar' the regular expression ` c*r' matches any of the strings` cr', ` car', ` cdr', ` caar', ` cadddar' and all othersuch strings with any number of ` a''s and ` d''s. The simplest case is onethat describes a particular string for example, the string ` foo' whenregarded as a regular expression matches ` foo' and nothing else.Nontrivial regular expressions use certain special constructs so that theycan match more than one string.
REGULAR EXPRESSION NOT A NUMBER FREE
The following section is taken (with minor modifications) from the GNU regularexpression library documentation, and is Copyright (C) Free SoftwareFoundation.Ī regular expression describes a set of strings.
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Regular expressions are a powerful way of specifying complex searchand replace operations.