Advanced Use


A .config attribute on the matcher is used to specify options. This means that you can explore the available options at the Python prompt (and perhaps via some IDEs).

For example:

>>> dir(matcher.config)
[..., 'add_monitor', 'add_rewriter', 'add_stream_kargs', 'alphabet',
'auto_memoize', 'cache_level', 'changed', 'clear', 'clear_cache',
'compile_to_dfa', 'compile_to_nfa', 'compile_to_re', 'compose_transforms',
'configuration', 'default', 'direct_eval', 'flatten', 'full_first_match',
'left_memoize', 'lexer', 'lines', 'low_memory', 'matcher',
'no_compile_to_regexp', 'no_compose_transforms', 'no_direct_eval',
'no_flatten', 'no_full_first_match', 'no_lexer', 'no_memoize',
'no_optimize_or', 'no_set_arguments', 'optimize_or', 'record_deepest',
'remove_all_monitors', 'remove_all_rewriters', 'remove_all_stream_kargs',
'remove_rewriter', 'right_memoize', 'set_alphabet_arg', 'set_arguments',
'stream_factory', 'trace_stack', 'trace_variables']
>>> help(matcher.config.compile_to_dfa)
Help on method compile_to_dfa in module lepl.core.config:
compile_to_dfa(self, force=False, alphabet=None) method of lepl.core.config.ConfigBuilder instance
    Compile simple matchers to DFA regular expressions.  This improves
    efficiency but may change the parser semantics slightly (DFA regular
    expressions do not provide backtracking / alternative matches).

Also, note that configuration methods can be chained together, making configuration code more compact:

>>> matcher.config.clear().lexer()

The main options available are described in the following sections. In addition, this example shows the effect of different options on parsing times.

Common, Packaged Actions


This sets the default configuration. It is not needed when first using a matcher, but can be useful to “reset” a matcher to the default state.

The default configuration is intended for safe, simple use. It can sometimes be made more efficient by calling .config.no_memoize(), at the risk of infinite loops with left-recursive grammars.


This empties the current configuration (for example, removing the default settings). If this is not used then any alterations are relative to the default settings.


Enable line aware parsing. This adds tokens that indicate line start and end points. See Line–Aware Parsing and the Offside Rule.

.config.lexer() .config.no_lexer()

Detect the use of Token() and modify the parser to use the lexer. Typically this is called indirectly via .config.default() (above).

Debug Actions

.config.full_first_match() .config.no_full_first_match()

Enable or disable the automatic generation of an error if the first match fails.


Add a monitor that works with the TraceVariables() context to show how matchers bind to values. Very useful and included by default.


Add a monitor to trace stack use (lots of complex output; not very useful). See TraceStack(). Removed by .config.remove_all_monitors() or .config.clear().


Add a monitor to record deepest match. See RecordDeepest(). Removed by .config.remove_all_monitors() or .config.clear().

Optimisation Actions

.config.flatten() .config.no_flatten()

Combined nested And() and Or() matchers.

Nested matchers typically occur because each & and | operator generates a new matcher, so a sequence of matchers separated by &, for example, generates several And() functions. This rewriter moves them into a single matcher, as might be expected from reading the grammar. This should not change the “meaning” of the grammar or the results returned and is included by default.

.config.compile_to_dfa() .config.compile_to_nfa() .config.compile_to_re() .config.no_compile_to_regexp()

Compile simple matches to regular expressions.

There are various restrictions about which matchers can be translated to regular expressions. The most important are that regular expressions cannot include recursive loops or transformations. So rewriting of regular expressions is typically restricted to those parts of the parser that recognise individual words.


.config.compile_to_dfa() may affect the parser semantics because the DFA engine does not support backtracking.


.config.compile_to_re() uses the Python re library, which cannot handle streams of data in the same way as Lepl. This means that matching using that library is restricted to strings only and does not support backtracking.

.config.optimize_or() .config.no_optimize_or()

Rearrange arguments to Or() so that left-recursive matchers are tested last. This improves efficiency, but may alter the parser semantics (the ordering of multiple results with ambiguous grammars may change).

The conservative parameter supplied to this rewriter indicates how left–recursive rules are detected. If true, all recursive paths are assumed to be left recursive. If false then only those matchers that are in the left–most position of multiple arguments are used (except for Or()).

.config.direct_eval() .config.no_direct_eval()

Combine simple matchers so that they are evaluated without trampolining. This is included by default.

.config.compose_transforms() .config.no_compose_transforms()

Combine transforms (functions applied to results) with matchers.

The Transform() matcher is the “workhorse” that underlies Apply(), >, etc. It changes the results returned by other functions.

Because transforms are not involved in the work of matching — they just modify the final results — the effects of adjacent instances can be combined into a single operation. In some cases they can also be merged into the operation of another matcher. This is done by the ComposeTransforms() rewriter.

These operations should not change the “meaning” of the grammar or the results returned, but should improve performance by reducing the amount of Trampolining made by the parser. They are included by default.

.config.auto_memoize() .config.left_memoize() .config.right_memoize() .config.no_memoize()

Remember previous inputs and results for matchers so that work is not repeated. See Memoisation.


Reduce memory use by explicitly managing resources and discarding old generators. See GeneratorManager(). Removed by .config.remove_all_monitors() or .config.clear().

While this will reduce memory use it also restricts backtracking and may mean that some inputs cannot be matched.


Control when streams are retained for debugging output. This is called by .config.low_memory() when appropriate (the streams can provide useful diagnostics, but increase memory use).

Low Level Actions

These methods are used internally. They may also be useful if you are developing a completely new functionality that is not supported by the “higher level” actions described above.

.config.add_rewriter() .config.remove_rewriter() .config.remove_all_rewriters()

Add or remove a rewriter, or remove all rewriters (possibly of a given type). Rewriters manipulate the matchers before the parser is used. This allows Lepl to use some of the techniques that make “compiled” parsers more efficient — but it can also introduce quite subtle errors. The addition of user–defined rewriters is not encouraged unless you are very familiar with Lepl.

.config.add_monitor() .config.remove_all_monitors()

Add a monitor, or remove all monitors. Monitors implement a callback interface that receives information about how Lepl is working. They can be used to share state across matchers, or to generate debugging information, for example.


Set the stream factory. This changes the class used to generate the stream for the parser, given some input (for example, matcher.parse_string() will call the from_string() method on this factory, to convert the string into a suitable stream).

.config.add_stream_kargs() .config.remove_all_stream_kargs()

Add additional arguments that are passed to the stream factory.


Set the alphabet, used by rgegular expressions. The default alphabet is suitable for Unicode data.

Argument Actions

Sometimes the same argument must be set on many matchers. Rather that setting each matcher individually, it is possible to set them all, via the configuration. These are used internally, to implement packaged actions; end-users should not need to call these methods in “normal” use.

.config.set_arguments() .config.no_set_arguments()

Set an argument, or clear all such settings.


Set the alphabet=... argument. If no value is given then the value given earlier to .config.alphabet() (or, if no value was given, the default Unicode alphabet) is used.

Search and Backtracking

Since Lepl supports full backtracking via generators it is possible to request all the alternative parses for a given input:

>>> from lepl import *

>>> any = Any()[:,...]
>>> split = any & any & Eos()
>>> match = split.match_string()

>>> [pair[0] for pair in match('****')]
[['****'], ['***', '*'], ['**', '**'], ['*', '***'], ['****']]

This shows that successive parses match less of the input with the first matcher, indicating that the matching is greedy.

Non-greedy (generous?) matching is achieved by specifying an array slice increment of 'b' (or BREADTH_FIRST):

>>> any = Any()[::'b',...]
>>> split = any & any & Eos()
>>> list(split.parse_all('****'))
[['****'], ['*', '***'], ['**', '**'], ['***', '*'], ['****']]

The greedy and non–greedy repetitions are implemented by depth (default, 'd', or DEPTH_FIRST), and breadth–first searches ('b' or BREADTH_FIRST), respectively.

In addition, by specifying a slice increment of 'g' (GREEDY), you can request a guaranteed greedy match. This evaluates all possibilities, before returning them in reverse length order. Typically this will be identical to depth–first search, but it is possible for backtracking to produce a longer match in complex cases — this final option, by evaluating all cases, re–orders the results as necessary.

Specifying 'n' (NON_GREEDY) gets the reverse ordering.

The tree implicit in the descriptions “breadth–first” and “depth–first” is not the AST, nor the tree of matchers, but a tree based on matchers and streams. In the case of a single, repeated, match this is easy to visualise: at any particular node the child nodes are generated by applying the matcher to the various streams returned by the current match (none if this is a final node, one for a simple match, several if the matcher backtracks).

So far so good. Unfortunately the process is more complicated for And() and Or().

In the case of And(), the first matcher is matched first. The child nodes correspond to the various (with backtracking) results of this match. At each child node, the second matcher is applied, generating new children. This repeats until the scope of the And() terminates at a depth in the tree corresponding to the children of the last matcher. Since And() fails unless all matchers match, only the final child nodes are possible results. As a consequence, both breadth and depth first searches would return the same ordering. The And() match is therefore unambiguous and the implementation has no way to specify the (essentially meaningless) choice between the two searches.

In the case of Or() we must select both the matcher and the result from the results available for that matcher. A natural approach is to assign the first generation of children to the choice of matcher, and the second level to the choice of result for the (parent) matcher. Again, this results in no ambiguity between breadth and depth–first results.

However, there is also an intuitively attractive argument that breadth–first search would return the first results of the different matches, in series, before considering backtracking. At the moment I do not see a “natural” way to form such a tree, and so this is not implemented. Feedback is appreciated.

Excluding Matches

It is also possible to exclude certain matches. This does not improve efficiency (the excluded matches have to be made anyway), but can simplify the logic of a complex parser.

The Difference() matcher takes two matchers as arguments. The first is matched as normal, but any matches that would also have been matched by the second matcher are excluded.

A good example, is the emulation of Float() using Real() and Integer() (remember that Real() matches both float and integer values):

>>> myFloat = Difference(Real(), Integer())
>>> list(myFloat.parse_all('1.2'))
[['1.2'], ['1.']]
>>> list(Real().parse_all('1.2))
[['1.2'], ['1.'], ['1']]


A memoizer stores a matcher’s results. If it is called again in the same context (during backtracking, for example), the stored result can be returned without repeating the work needed to generate it. This can improve the efficiency of the parser.

Lepl 2 has two memoizers. The simplest is RMemo() which is a simple cache based on the stream supplied.

For left–recursive grammars, however, things are more complicated. The same matcher can be called with the same stream at different “levels” of recursion (for full details see Memoisation). In this case, LMemo() must be used.

Memoizers can be specified directly in the grammar or they can be added via several configuration options, described below.

When added directly to the grammar a memoizer only affects the given matcher(s). For example:

>>> matcher = Any('a')[:] & Any('a')[:] & RMemo(Any('b')[4])
>>> len(list(matcher.match('aaaabbbb')))

Here the RMemo() avoids re-matching of the “bbbb”, but has no effect on the matching of the “a”s.

To explicitly apply a memoizer to all matchers use .config.left_memoize() or .config.right_memoize():

>>> class VerbPhrase(Node): pass
>>> class DetPhrase(Node): pass
>>> class SimpleTp(Node): pass
>>> class TermPhrase(Node): pass
>>> class Sentence(Node): pass

>>> verb        = Literals('knows', 'respects', 'loves')         > 'verb'
>>> join        = Literals('and', 'or')                          > 'join'
>>> proper_noun = Literals('helen', 'john', 'pat')               > 'proper_noun'
>>> determiner  = Literals('every', 'some')                      > 'determiner'
>>> noun        = Literals('boy', 'girl', 'man', 'woman')        > 'noun'

>>> verbphrase  = Delayed()
>>> verbphrase += verb | (verbphrase // join // verbphrase)      > VerbPhrase
>>> det_phrase  = determiner // noun                             > DetPhrase
>>> simple_tp   = proper_noun | det_phrase                       > SimpleTp
>>> termphrase  = Delayed()
>>> termphrase += simple_tp | (termphrase // join // termphrase) > TermPhrase
>>> sentence    = termphrase // verbphrase // termphrase & Eos() > Sentence

>>> p = sentence.left_memoize()
>>> len(list(p('every boy or some girl and helen and john or pat knows '
>>>            'and respects or loves every boy or some girl and pat or '
>>>            'john and helen')))

This example is left–recursive and very ambiguous. With LMemo() added to all matchers it can be parsed with no problems.

Because left–recursive grammars can be very inefficient, and because Lepl’s support for them has historically been unreliable (buggy), they are no longer (since Lepl 5) supported by default. Instead, RMemo() is added, which can detect left–recursion and print a suitable warning.