{"id":14860,"date":"2019-05-28T19:22:29","date_gmt":"2019-05-28T17:22:29","guid":{"rendered":"https:\/\/www.roumazeilles.net\/news\/en\/wordpress\/?page_id=14860"},"modified":"2020-11-24T21:27:11","modified_gmt":"2020-11-24T19:27:11","slug":"regular-expressions","status":"publish","type":"page","link":"https:\/\/www.roumazeilles.net\/news\/en\/wordpress\/shareware-home\/regular-expressions\/","title":{"rendered":"Regular expressions"},"content":{"rendered":"\n

Search regular expressions<\/h3>\n\n\n\n

The regular expression routines of the YGrep Search Engine support a full range of Unix regular expressions as defined in ed(1) and in grep(1) . They are also very similar to regular expressions provided by the Emacs or MicroEmacs text editor or Borland utility GREP.COM utility.<\/p>\n\n\n\n

Specification<\/h4>\n\n\n\n
\n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n
| <\/TD>\n A vertical bar between expressions forces matches onto either the \n first expressions OR the second expression. Up to 10 of these can \n be combined. <\/TD>\n <\/TR>\n
& <\/TD>\n An ampersand between expressions forces matches onto both the first \n expression AND the second expression. Up to 10 of these can be combined. \n <\/TD>\n <\/TR>\n
^ <\/TD>\n A circumflex as the first character of the pattern forces matches \n to beginning of lines. <\/TD>\n <\/TR>\n
$ <\/TD>\n A dollar as the last character of the pattern forces matches to \n end of lines. <\/TD>\n <\/TR>\n
. <\/TD>\n A period anywhere in the string matches any single character. <\/TD>\n <\/TR>\n
* <\/TD>\n An expression followed by an asterisk matches zero or more occurrences \n of that expression. <\/TD>\n <\/TR>\n
+ <\/TD>\n An expression followed by a plus sign matches one or more occurrences \n of that expression. <\/TD>\n <\/TR>\n
– <\/TD>\n An expression followed by a minus sign optionally matches that expression \n (matches zero or one occurrence of that expression). <\/TD>\n <\/TR>\n
{} <\/TD>\n An expression followed by a number N in curly braces matches the \n expression N times.<\/TD>\n <\/TR>\n
{} <\/TD>\n An expression followed by two numbers M and N separated by a comma \n in curly braces matches the expression M to N times. See the examples.<\/TD>\n <\/TR>\n
[] <\/TD>\n A string enclosed in square brackets matches any character in that \n string, but no others. If the first character of the string is a circumflex \n the expression matches any character except the characters in the \n string. A range of characters may be specified by two characters separated \n by a -. These are known as character classes. <\/TD>\n <\/TR>\n
\\< <\/TD>\n A backslash followed by an opening < matches the beginning of a \n word. <\/TD>\n <\/TR>\n
\\> <\/TD>\n A backslash followed by a closing > matches the end of a word. <\/TD>\n <\/TR>\n
\\( <\/TD>\n A backslash followed by an opening ( describes the beginning of \n a tagged sub-expression (see Search and replace regular expressions, \n it has no effect on search-only expressions). No more than 9 sub-expressions \n are allowed by the YGrep Search Engine. <\/TD>\n <\/TR>\n
\\) <\/TD>\n A backslash followed by a closing ) describes the end of a tagged \n sub-expression (see Search and replace regular expressions, it has \n no effect on search-only expressions). No more than 9 sub-expressions \n are allowed by the YGrep Search Engine. <\/TD>\n <\/TR>\n
\\b <\/TD>\n A backslash followed by a letter ‘b’ matches the backspace character \n (ASCII code 8). <\/TD>\n <\/TR>\n
\\n <\/TD>\n A backslash followed by a letter ‘n’ matches the newline character \n (ASCII code 10). <\/TD>\n <\/TR>\n
\\f <\/TD>\n A backslash followed by a letter ‘f’ matches the form-feed character \n (ASCII code 12). <\/TD>\n <\/TR>\n
\\r <\/TD>\n A backslash followed by a letter ‘r’ matches the carriage return \n character (ASCII code 13). <\/TD>\n <\/TR>\n
\\t <\/TD>\n A backslash followed by a letter ‘t’ matches the horizontal tab \n character (ASCII code 9). <\/TD>\n <\/TR>\n
\\x00 <\/TD>\n A backslash followed by a letter ‘x’ and a hexadecimal code matches \n the character with that hexadecimal ASCII code. <\/TD>\n <\/TR>\n
\\ <\/TD>\n A backslash followed by any other character quotes that character. \n This allows a search for a character that is usually a regular expression \n specifier. <\/TD>\n <\/TR>\n <\/TABLE>\n <\/figure>\n\n\n\n

Important note to the users of the C language:<\/h4>\n\n\n\n

C programmers should remember that in C strings, a backslash is a special character and it should be \ufffddoubled\ufffd in order to be inserted in the C-declared constant strings. The external user will use the backslash character as described here. For example in a C source program the expression “\\<hello\\>” should be written as the C string “\\\\<hello\\\\>”. The error is very common (I do it myself at regular intervals) and may make you scratch you head in front of a bizarre bug.<\/p>\n\n\n\n

If an enclosure must contain either the dash (‘-‘) or the closing bracket (‘]’), these characters must appear at the beginning of the enclosure list like in “[]-]”. Please note that ‘]’ must appear before ‘-‘.<\/ins><\/p>\n\n\n\n

Examples<\/h4>\n\n\n\n
\n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n \n
foo|bar|toto <\/TD>\n matches one of the words: foo, or bar, or toto <\/TD>\n <\/TR>\n
foo&bar&toto <\/TD>\n matches all of the words: foo, and bar, and toto <\/TD>\n <\/TR>\n
^Windows <\/TD>\n matches all lines starting with Windows <\/TD>\n <\/TR>\n
Grep$ <\/TD>\n matches all lines ending with Grep <\/TD>\n <\/TR>\n
\\$ <\/TD>\n matches a dollar sign <\/TD>\n <\/TR>\n
H..p <\/TD>\n matches Help, Hoop, Harp, etc. <\/TD>\n <\/TR>\n
H.*p <\/TD>\n matches Help, Hoop, Harp, etc. but also fragments beginning with \n H and finishing with p like Heeellllp, Holy crop, Halt and stop, etc. \n <\/TD>\n <\/TR>\n
^W.n <\/TD>\n matches all lines starting with Win, Wan, Won, etc. <\/TD>\n <\/TR>\n
fo* <\/TD>\n matches f, fo, foo, etc. <\/TD>\n <\/TR>\n
fo+ <\/TD>\n matches fo, foo, etc. <\/TD>\n <\/TR>\n
fo-<\/TD>\n matches f, fo, but not foo, fooo, etc. <\/TD>\n <\/TR>\n
[xyz] <\/TD>\n matches x, y and z <\/TD>\n <\/TR>\n
a[^xyz]c <\/TD>\n matches abc, arc and aXb but not axb <\/TD>\n <\/TR>\n
([0-9]) <\/TD>\n matches (0), (1), (2), (3), (4), (5), (6), (7), (8) and (9) <\/TD>\n <\/TR>\n
([0-9]*) <\/TD>\n matches (), (0), (123), (2512), etc. <\/TD>\n <\/TR>\n
\\<[Aa].*\\> <\/TD>\n matches any non-empty word beginning with either a or A like A, \n Ab, Abc, a, abC, etc. <\/TD>\n <\/TR>\n
a{2} <\/TD>\n matches two (2) characters a <\/TD>\n <\/TR>\n
a{2,4} <\/TD>\n matches 2, 3 or 4 characters a <\/TD>\n <\/TR>\n <\/TABLE><\/figure>\n\n\n\n

About the order of precedence of the | and & operators:<\/h4>\n\n\n\n

You should be aware of the precedence that is implied by the Boolean operators inside a regular expression. The expression is “explored” from the beginning (from the left). The precedence can be easily considered recognized by saying that a YGrep Search Engine regular expression containing Boolean operators is implicitly parenthesized to the end (to the right). For example:<\/p>\n\n\n\n

Expr1 | Expr2 & Expr3 & Expr4 | Expr5<\/pre>\n\n\n\n
is equivalent to the grouped theoretical expression:<\/p>\n\n\n\n
Expr1 | ( Expr2 & ( Expr3 & ( Expr4 | ( Expr5 ))))<\/pre>\n\n\n\n
Consequently, the exact order of the sub-expressions can be quite important to reach the exact objective of the programmer.<\/p>\n\n\n\n
What is the matched with & operator:<\/h4>\n\n\n\n
The matched string (as returned by the TagStart[0] and TagEnd[0] of the RGrep() RGREPINFO returned value) is a little specific when you use the AND-operator (&) in a regular expression. As a matter of fact, the matched string is the shortest string containing all of the matched string for each of the individual searches.<\/p>\n\n\n\n
For example, foo&bar will match on the following line:<\/p>\n\n\n\n
Test1 bar schwoop foo test2<\/pre>\n\n\n\n
The matched string returned in (as returned by the TagStart[0] and TagEnd[0] of the RGrep() RGREPINFO returned value) is the string containing “bar schwoop foo”.<\/p>\n\n\n\n
Even if this has no specific relation to how it is implemented, in this simple example, the expression foo&bar was equivalent to bar.*foo|foo.*bar, and both match the same string.<\/p>\n\n\n\n
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Search-and-Replace regular expressions<\/h3>\n\n\n\n
The YGrep Search Engine regular expression substitution routines support a small set of expressions to define how the substitution will be performed.<\/p>\n\n\n\n
Specification<\/h4>\n\n\n\n
      \n         \n           \n           \n           \n          
& <\/TD>\n          An ampersand in the substituted string forces insertion of the full \n            matched pattern. <\/TD>\n        <\/TR>\n        
\\number <\/TD>\n          A backslash followed by a number (between 1 and 9) forces the insertion \n            of the tag matched with the equivalent number in the pattern. <\/TD>\n        <\/TR>\n        
\\0 <\/TD>\n          A backslash followed by a 0 forces the insertion of the the full \n            matched pattern (like &). <\/TD>\n        <\/TR>\n        
\\& <\/TD>\n          is an escape sequence to allow the insertion of the & character \n            (while removing its matched pattern meaning). <\/TD>\n        <\/TR>\n      <\/TABLE><\/figure>\n\n\n\n
Examples<\/h4>\n\n\n\n
Search Pattern\tReplace Pattern\tSubstitution\nWindows\tMS-&\treplaces all occurrences of the simple word Windows with MS-Windows\n\\(dows\\)\\([Ww]in\\)\t\\2\\1\tallows to reorder the pattern dowsWin into the normal Windows regardless of the letter-casing of the W in the beginning of the word<\/figure>\n\n\n\n
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Other interesting links<\/h2>\n\n\n\n
General links<\/h3>\n\n\n\n