Match a [regular expression] against a string : '''regexp''' ''?switches? exp string ?matchVar? ?subMatchVar subMatchVar ...? '' http://www.purl.org/tcl/home/man/tcl8.5/TclCmd/regexp.htm **DESCRIPTION** Determines whether the regular expression ''exp'' matches part or all of ''string'' and returns 1 if it does, 0 if it doesn't. (Regular expression matching is described in the [re_syntax] reference page.) If additional arguments are specified after ''string'' then they are treated as the names of variables in which to return information about which part(s) of ''string'' matched ''exp''. ''MatchVar'' will be set to the range of ''string'' that matched all of ''exp''. The first ''subMatchVar'' will contain the characters in ''string'' that matched the leftmost parenthesized subexpression within ''exp'', the next ''subMatchVar'' will contain the characters that matched the next parenthesized subexpression to the right in ''exp'', and so on. If the initial arguments to '''regexp''' start with '''-''' then they are treated as switches. The following switches are currently supported: '''-about''': Instead of attempting to match the regular expression, returns a list containing information about the regular expression. The first element of the list is a subexpression count. The second element is a list of property names that describe various attributes of the regular expression. This switch is primarily intended for debugging purposes (see '''REGEXP DESCRIPTIVE FLAGS''' below). '''-expanded''': Enables use of the expanded regular expression syntax where whitespace and comments are ignored. This is the same as specifying the `(?x)` embedded option (see METASYNTAX, below). '''-indices''': Changes what is stored in the ''subMatchVars''. Instead of storing the matching characters from string, each variable will contain a list of two integers giving the indices in string of the first and last characters in the matching range of characters. '''-line''': Enables newline-sensitive matching. By default, newline is a completely ordinary character with no special meaning. With this flag, ‘`[[^`’ bracket expressions and ‘`.`’ never match newline, ‘`^`’ matches an empty string after any newline in addition to its normal function, and ‘`$`’ matches an empty string before any newline in addition to its normal function. This flag is equivalent to specifying both '''-linestop''' and '''-lineanchor''', or the `(?n)` embedded option (see METASYNTAX, below). '''-linestop''': Changes the behavior of ‘`[[^`’ bracket expressions and ‘`.`’ so that they stop at newlines. This is the same as specifying the `(?p)` embedded option (see METASYNTAX, below). '''-lineanchor''': Changes the behavior of ‘`^`’ and ‘`$`’ (the “anchors”) so they match the beginning and end of a line respectively. This is the same as specifying the `(?w)` embedded option (see METASYNTAX, below). '''-nocase''': Causes upper-case characters in string to be treated as lower case during the matching process. '''-start''' ''index'': Specifies a character index offset into the string to start matching the regular expression at. When using this switch, ‘`^`’ will not match the beginning of the line, and \A will still match the start of the string at index. If '''-indices''' is specified, the indices will be indexed starting from the absolute beginning of the input string. index will be constrained to the bounds of the input string. '''--''': Marks the end of switches. The argument following this one will be treated as ''exp'' even if it starts with a `-`. If there are more ''subMatchVar'''s than parenthesized subexpressions within ''exp'', or if a particular subexpression in ''exp'' doesn't match the string (e.g. because it was in a portion of the expression that wasn't matched), then the corresponding ''subMatchVar'' will be set to "-1 -1" if '''-indices''' has been specified or to an empty string otherwise. (From: [TclHelp] 8.2.3) **EXAMPLE** ====== puts "enter string:" set input [read stdin] if {[regexp "abc" $input]} { puts "yes" } else { puts "no" } ====== **REGEXP DESCRIPTIVE FLAGS** More info about the return values from ''-about'', written by [DKF] in Feb, 2007 ''(with further additions and clarifications by DKF from a bit later in italics)'': " currently only exist for testing purposes. Going through the definitive list, I see: REG_UBACKREF: Indicates that the RE contains backreferences, which forces a more expensive evaluation engine. (Note that this implies that there must be capturing parens, but there is no flag to indicate that.) ''A simple RE that has this flag set: (.)\1'' REG_ULOOKAHEAD: Indicates that the RE contains lookahead constraints. ''A simple RE that has this flag set: foo(?=bar)'' REG_UBOUNDS: Indicates that the RE contains bounded matches (i.e. counted ranges expressed in the form {m,n}). ''A simple RE that has this flag set: [[a-c]]{3,5}'' REG_UBRACES: Indicates that the RE contains braces that are not bounds. ''A simple RE that has this flag set: a{}b'' REG_UBSALNUM: Indicates that there's a would-be rich backslash-alphanumeric sequence. Only happens when switched to parsing non-advanced REs. ''A fairly-simple RE that has this flag set: (?e)\a'' REG_UPBOTCH: Indicates an unbalanced close-parenthesis ("specification botch" according to a comment in the source!) ''A fairly-simple RE that has this flag set: (?e))'' REG_UBBS: Indicates that there is a backslash inside a bracketed character set. ''A simple RE that has this flag set: [[\w'?!.]]'' REG_UNONPOSIX: Indicates that the RE is not a POSIX RE. ''Happens a lot! The POSIX RE spec is restrictive.'' REG_UUNSPEC: Indicates that the RE is asking for unspecified behaviour? ''It's not clear what "unspecified" really means here.'' REG_UUNPORT: Indicates that the RE is unportable? ''Portable to what? I don't know.'' REG_ULOCALE: Indicates that the RE is (potentially) dependent on the locale. ''Many sets of characters theoretically depend on the locale, but Tcl only actually has a single locale for REs so this is really a pointless RE gnosticism.'' REG_UEMPTYMATCH: Indicates that the empty string is matched by the RE. ''A simple RE that has this flag set: .*'' REG_UIMPOSSIBLE: Indicates that the RE cannot possibly match anything. (Not all "impossible" REs are detected though.) ''A fairly-simple RE that has this flag set: foo\m'' REG_USHORTEST: Indicates that the RE is non-greedy, and so uses a different matching engine. ''A simple RE that has this flag set: a.??b'' If you're not an RE wonk or matcher, I'd assert that virtually all of these are totally uninteresting. :-) The backrefs, lookahead and bounds are probably most interesting from a "describing what's in there" POV." ''I can't see any value in UNONPOSIX, UUNSPEC, UUNPORT or ULOCALE; they just don't seem to correspond to any question I might ever wish to ask about a regular expression. UBSALNUM and UPBOTCH are very low-value too, as they only apply when you move the RE engine into a non-standard mode.'' <> **METASYNTAX, anyone?** [re_syntax] covers the regular expression syntax, right? ---- **How do you use regexp to perform regular expression based comparisons?** **How do you use regexp to perform regular expression based string replacements?** You don't use regexp for replacments - see [regsub] for that. ---- **Greedy vs Non-greedy** someone needs to write up greedy vs non-greedy re issues [MG] OK, I'm sure someone can do this better than me, but since nothing's here at the moment I'll make a start... By default, the regexp characters ''+'' and ''*'' match as much as possible (which is called greedy matching). By placing a ''?'' after them, you can make them match as little as possible (non-greedy). For example... regexp "a.+3" "abc123abc123" var set var would show the match as ''abc123abc123'', because the + is matches all the characters up until the last 3. If you used... regexp "a.+?3" "abc123abc123" var set var you'd see the match as ''abc123'' because +? matches as little as possible. Greedy regexp matching is a particular problem in parsing HTML, etc, because... set str "Some Bold Text
Some Italic Text
More Bold Text" regexp "(.*)" $str -> var set var would show ''Some Bold Text
Some Italic Text
More Bold Text'' - matching as much as possible, it takes between the first occurance of and the ''last'' occurance of . But, using a non-greedy regexp to match... set str "Some Bold Text
Some Italic Text
More Bold Text" regexp "(.*?)" $str -> var set var would show what you want; ''Some Bold Text''. Hope that explanation/rambling is some use, at least until someone with more idea what they're doing puts something up :) [AvL] I'll now mention some common pitfall with non-greedy REs: Lets go back to the first example, but with a modified string: regexp "a.+?3" "abc123ax3" var set var Although second possible match ''ax3'' would be shorter, it will still find the first match ''abc123'', because even with non-greedy quantifiers, the first match always wins. ---- **Questions** ***RE method to deal with newline/carriage return*** Could someone replace this line with some verbage regarding the way one uses regular expressions for specific newline carriage return handling (as opposed to the use of the $ metacharacter)? [Janos Holanyi]: I would really need to build up a re that would match one line and only one line - that is, excluding carriege-return-newline's (\r\n) from matching... How would such a re look like? ---- [LV] how about something like this? set a "abc dev" # a now has two lines in it regexp -line -- {(.*)} $a b c d 1 puts $b abc puts $c abc Note that if you want to keep carriage returns or newlines by themselves, but not when they are together, you need something like: regexp -- {^([^\r]|\r(?!\n))*} $a b c d This allows plain carriage return or plain newline. Thanks to [bbh] and [Donal Fellows] for this regular expression. ---- ***Using back references*** From [the comp.lang.tcl newsgroup]: I did some experimenting with other strings, like "just a HHHHEEEEAAAADDDDEEEERRRR". The regular expression {(.)\1\1\1} does the job I would have wanted, whereas {(.){4}} will return the last of each four characters - as posted as well. That surprised me too -- being able to place backreferences within the regex is an extremely powerful technique. regsub -all {(.)\1{3}} $string {\1} result for exactly 4 char repeats, and {(.)\1+} for arbitrary repeats ---- ***Tool for visualing regular expression operation*** [Laurent Riesterer] has written a Visual Regexp tool [http://laurent.riesterer.free.fr/regexp] to help understand regexp operation. ---- ***How to use quotes and variables in regular expressions?*** Feb 9th 2007 CJL wondered on Ask#5 what the correct/best/proper way of writing a regexp with quotes and the current value of a variable in the expression was? I want to match various patterns of the form , where $something has a range of values that is a subset of all possible values, i.e. I don't want to put \S+ in place of $something as that will give unwanted matches. Note the presence of quotes and escapes to complicate things. [MG] Using [format] is probably one of the simplest. set something "foobar" set pattern {} set pattern [format $pattern $something] Assuming, of course, you don't have %-'s in your string. Otherwise, building it in steps may be easiest: set something "foobar" set pattern {} [LV] I suspect the OP will need to replace those \d with %d and the \S with %s . ---- ***How to match anything but a specific string?*** [NEM] - A question on the Tclers Chat brought up a common problem that I've had when dealing with regular expressions. The RE engine allows [[^AB]] to mean "not A or B", but what if you want to match anything but the string "AB"? The only way to do it is to put lots of negated classes one after the other, which is ugly. So, here is a way to wrap that up into something a bit more elegant: proc not {pattern} { set ret "(?:" ;# Not capturing bracket foreach char [split $pattern {}] { append ret "\[^$char\]" } append ret ")" return $ret } Then you can do: regexp -- "AB([not AB]*)AB(.*)" ABcdefghABijklmnopqrst -> first rest first = "cdefgh" rest = "ijklmnopqrst" And it handles things like: regexp -- "AB([not AB]*)AB(.*)" ABcdefghBAijkABlmnopqrst -> first ret first = "cdefghBAijk" rest = "lmnopqrst" Note though, that this will only match patterns which are at least the same length as the negated expression: regexp -- "AB([not AB]*)AB(.*)" ABcABslkdjf -> first rest => 0 The proper solution to this problem is a lot more complex, unfortunately. ---- The above three regexp's can be written using a lookahead constraint. foreach str {ABcdefghABijklmnopqrst ABcdefghBAijkABlmnopqrst ABcABslkdjf} { set e "regexp -- {AB(?!AB)(.*)AB(.*)} $str -> first rest" puts "$e\n=> [eval $e]\nfirst = $first\nrest = $rest\n" } Output: regexp -- {AB(?!AB)(.*)AB(.*)} ABcdefghABijklmnopqrst -> first rest => 1 first = cdefgh rest = ijklmnopqrst regexp -- {AB(?!AB)(.*)AB(.*)} ABcdefghBAijkABlmnopqrst -> first rest => 1 first = cdefghBAijk rest = lmnopqrst regexp -- {AB(?!AB)(.*)AB(.*)} ABcABslkdjf -> first rest => 1 first = c rest = slkdjf ---- Important to note on the "[[not pattern]]" example above is that it will NOT match strings where there is an occurrence of the first letter from {pattern} when not part of the entirety of {pattern}: % regexp -- "AB([not AB]*)AB(.*)" ABcdefghAijklmABnopqrst -> first rest 0 % regexp -- "AB([not AB]*)AB(.*)" ABcdefghBijklmABnopqrst -> first rest 1 % set first cdefghBijklm % set rest nopqrst ---- [DKF]: It's actually fairly easy to request that an RE shouldn't match something. You just need some magic around it like this: regexp {^(?:(?!AB).)*$} $string That matches any string that doesn't contain "AB" as a subsequence. ---- ***Tcl variables containing compiled regular expressions*** [elfring] 2003-10-29 TCL variables can be marked that an instance contains a compiled regular expression. REs can be pre-compiled by the call "regexp $RE {}" [http://sourceforge.net/tracker/?group_id=10894&atid=360894&func=detail&aid=832230]. ---- ***Non-reporting sub-patterns*** I would love to see a some clarification on exactly how non-reporting subpatterns work with -inline, specifically if you can silence the overall pattern match: % set str { asd;flkj sad;lfjl;kjf asdflaj;lkfjasdf lsdk } % set _Img {]*>} ]*> % regexp -all -nocase -inline $_Img $str {} example.jpg {} browser/ie.gif [glennj]: You can't silence the full match. You will have to iterate over the results of regexp thusly: set matches [list] foreach {full submatch} [regexp -all -nocase -inline $_Img $str] { lappend matches $submatch } ---- ***Problem of needing to match optional parts with regular expressions*** [elfring] 2004-07-05 Does anybody know problems and solutions to match optional parts with regular expressions [http://groups.google.de/groups?group=comp.lang.tcl&selm=40ed1d8f.0407010130.3e899f5d%40posting.google.com]? [MG] July 17th 2004 - The problem with the regexp there seems to be that one of the parts to match optional white space is in the wrong place, and is matching too much. If you use this regexp instead, it works for me, on Win XP with Tcl 8.4.6. (The change is that, after and before , the ''.*?'' has been moved inside the (?: ... ) set pattern {(.+)(?:.*?(SYSTEM|PUBLIC).*?(.+)(?:.*?(.+))?)?(?:.*?(.*?))?(?:.*?(.*?))?.*?(.*)\s*$} set string {gruss SYSTEM http://XXX/Hallo.dtd http://YYY/Leute.dtd Versuch="1"

Guten Tag!

} regexp $pattern $str z name scope system public definition attributes content ---- ***Regular Expression for parsing http string*** regexp {([[^:]]+)://([[^:/]]+)(:([[0-9]]+))} [ns_conn location] match protocol server x port the above author should remember this is a TCL wiki, and not an [aolserver] one, but thanks for the submission ;) ---- ***Regular Expressions Caching*** Tcl dynamically caches the compiled regular expressions. The Tcl core caches the last 30 REs it compiled but you can cause an number of RE's to be cached by assigning them to variables. If a regular expression is assigned to a variable and the variable is not changed, the Tcl core will save the compiled version of the RE and use the precompiled version of the variable during next evaluation. In the core the compiled version of the RE is stored in the Tcl_Obj, along with its string representation. To find #pragma statements define a pattern like set re {^\s*#\s*pragma\s+(.)} if { [regexp $re $line -> rest] } { ... } The above example will cause the compiled regular expression to be stored in the '''re''' variable. (From c.l.t [http://groups.google.com/groups?threadm=1l6lo01kqjm5vd0rl8b027p83rto660v7i%404ax.com]) The run time benefit of regular expression caching can easily be shown: # Run N different regexp patterns proc test_regexps N { for {set i 0} {$i < $N} {incr i} { regexp "foobar$i" "foobar1" } } puts "29 Took: [time { test_regexps 29 } 100]" puts "30 Took: [time { test_regexps 30 } 100]" puts "31 Took: [time { test_regexps 31 } 100]" puts "32 Took: [time { test_regexps 32 } 100]" One run of this gave: 29 Took: 298 microseconds per iteration 30 Took: 372 microseconds per iteration 31 Took: 2000 microseconds per iteration 32 Took: 2107 microseconds per iteration ...clearly showing the extra cost of having to recompile each regexp pattern each time thro' due to exceeding the NUM_REGEXPS (30). ---- ***Using Regular Expressions to Strip Visually Blank Lines*** ''[DKF] writes that'' it is hard to do this with any single RE on its own, though you can do it quite easily using a couple of things coupled together. This example uses [regsub] to strip the problematic lines, but cannot completely get rid of leading and trailing newlines without the extra [string trim]: string trim [regsub -all {\n(?:\s*\n)+} $data \n] \n However, I prefer selecting things positively, leading to a solution using [regexp] and [join]: join [regexp -all -inline {(?=[^\n]*\S)[^\n]+} $data] \n [DKF] 10-Aug-2006: More experimentation indicates that a single [regsub] can do the whole job: regsub -all {^\n+|\n+$|(\n)+} $data {\1} Note that the order of the alternatives is important! ---- ***A Regular Expression to Match Many Things in Any Order*** [DKF]: Sometimes it is useful to be able to write a regular expression that matches a string that contains some number of substrings (typically words) in any order. In normal regexps, this is a horrible thing to write down as the size of the RE term varies exponentially with the number of substrings. However, if you don't mind matching behaviour that is guaranteed to be non-optimal in some strict sense, and if you don't want ''any'' capturing parens, you can use positive lookahead assertions to make things neater. Thus, to match a string that contains '''foo''', '''bar''' and '''spong''' within it in any order, use a RE like this: set RE {(?=.*foo)(?=.*bar)(?=.*spong).} set matched [regexp $RE $string] Just note that if you use this, you ''cannot'' know where those strings matched; lookahead assertions don't support that. If you need that data, use multiple [regexp] matches instead ---- ***Looping over matches*** [MAH]: What yould be the correct way to loop over all matches of a regular expression in a string? I came up with the following solution for finding all include statements in a string, but using -start has side effects on the meaning of characters like $ and ^. set pos 0 while {[regexp -start $pos {`include "([\w/.]+)"} $data string vincfile]==1} { set pos [expr {$pos+[string length $string]}] puts "file=$vincfile" } [Lars H]: The option combination '''-all -inline''' is probably what you're looking for (although in general the problem of "finding all matches" runs into several technical issues, due to the fact that matches may overlap). In combination with -start, one has to use \A and \Z instead of $ and ^, unless the intent is to use the newline-sensitive behaviours of the latter. -indices may also be useful. [MAH]: Okay, -inline is too clumsy for me since I don't want the overall match string. Instead I'll go with -indices. This gives me set pos 0 while {[regexp -start $pos -indices {`include "([\w/.]+)"} $data -> vincfilepos]==1} { set vincfile [ string range $data [ lindex $vincfilepos 0 ] [ lindex $vincfilepos 1 ] ] set pos [ lindex $vincfilepos 1 ] puts "file=$vincfile" } I think that's rather clumsy for a task this common. Any ideas on how to make it simpler? ---- [BAS] : just a tidbit, the [Postgresql] DBMS uses Tcl's regexp engine for its own regexp handling; see [http://archives.postgresql.org/pgsql-announce/2003-02/msg00008.php]. ---- ***Counting occurrences*** '''Saravanan''': Can any one tell how to retrieve the count of a particular character from the given string (using regexp only)? Eg: `set a "hithisisisis"`. i need to find how many occurrences of 'i' from $a. [Lars H]: Use the -all option: % regexp -all i $a 5 <> ---- **See also** * [Regular Expression Examples] * [regsub] * [string match] * [glob] * [Mismatch between regexp -indices and switch -regexp -indexvar] * http://www.regexlib.com/ - a community maintained library of regular expressions for use <> Tcl syntax help | Arts and Crafts of Tcl-Tk Programming | Command | String Processing | Regular Expressions | Regular Expression Examples | Regular Expression Debugging Tips